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Soil Science Society of America Journal 66:848-856 (2002)
© 2002 Soil Science Society of America

DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Management Effects on Barley Straw Decomposition, Nitrogen Release, and Crop Production

M. H. Beare*, P. E. Wilson, P. M. Fraser and R. C. Butler

New Zealand Institute for Crop & Food Research, Ltd., Canterbury Agriculture and Science Centre, Private Bag 4704, Christchurch, New Zealand

* Corresponding author (Bearem{at}crop.cri.nz)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Development of sustainable-crop production systems depends on identifying effective strategies for the management of postharvest crop residues. The effects of time-of-incorporation (autumn-incorporated [AI] vs. spring-incorporated [SI] and irrigation (irrigated[Irr] vs. nonirrigated[Nirr]) on barley Hordeum vulgare L. straw decomposition and microbial activity were investigated in relation to soil N availability and crop production over one cropping cycle in Canterbury, New Zealand. Over the winter-fallow period, the weight loss of AI barley straw averaged 33% as compared with 18% for surface straw of SI treatments. By harvest, nearly all of the difference in mass loss between AI and SI straw (17%) from NIrr treatments could be attributed to decomposition in the fallow period. Irrigation increased straw decomposition during the cropping period by 68% in AI treatment compared with only 37% in SI treatment. The effect of winter-straw placement on the response of barley straw to summer irrigation was related to the size of the residue-borne microbial populations at the start of the cropping period. Although relatively little N was released (<5 kg N ha-1) from decaying barley straw, cultivation, and incorporation of straw in autumn (AI) did result in greater topsoil (0–25 cm) mineral N levels during the winter period as compared with the SI treatment. Overall, Irr and AI of straw increased the dry matter production and N uptake of the summer barley crop, resulting in a concomitant decrease in soil mineral N levels relative to Nirr SI treatments. The mechanisms that explain this difference in crop response to winter residue management require further investigation.

Abbreviations: AFDW, ash-free dry weight • AI, autumn-incorporated • Irr, irrigated • Nirr, nonirrigated • SI, spring-incorporated • SIR, substrate-induced respiration • LSD, least significant difference


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
EFFECTIVE MANAGEMENT OF postharvest crop residues remains an important issue in many grain-producing regions of the world. In those regions under intensive production, postharvest crop residues are often considered a waste product requiring disposal before production of another crop. However, the disposal of crop residues by removal (baling) or burning is often criticized for accelerating losses of soil organic matter and nutrients, increasing C emissions and reducing soil microbial activity (Biederbeck et al., 1980; Rasmussen et al., 1980; Cookson et al., 1998). While residue incorporation can help to minimize these impacts, it can also create impediments to cultivation (Dickey et al., 1994) and promote residue-borne diseases of crops (Cook and Haglund, 1991; Jenkyn et al., 1995). Incorporation of crop residues can also result in temporary nutrient limitations owing to microbial immobilization (Jenkinson, 1985; Addiscott and Dexter, 1994).

In the Canterbury region of New Zealand, cereal straw has traditionally been disposed of by burning or removal. Increasingly, New Zealand farmers are looking to straw incorporation as a strategy for improving soil organic matter management and allaying public concerns over the impacts of straw burning on air pollution. However, where residues have been incorporated, farmers often cite concerns for reduced fertility resulting from nutrient immobilization and difficulties in soil cultivation, problems that are attributable to slow rates of residue decay (Cookson et al., 1998). Effective mitigation of these effects depends on developing crop residue management strategies that enhance residue breakdown. Realizing the potential benefits of cereal straw incorporation depends on synchronizing nutrient release with the demands of the crop, while minimizing the risks to nutrient losses (Powlson et al., 1985; Jarvis et al., 1989; Bhogal et al., 1997).

In cool, moist temperate climatic zones, some farmers incorporate straw just prior to sowing in the spring (i.e., residues left on the soil surface over the preceding winter), while others incorporate straw soon after harvest in autumn. As compared with incorporated straw, surface straw is often exposed to greater fluctuations in temperature and moisture and lower nutrient availability (Douglas et al., 1980; Beare, 1997), all of which may reduce microbial activity and, hence, the rate of decomposition. Summer irrigation is likely to enhance the rate of decomposition and nutrient release, particularly when straw is incorporated (Douglas and Rickman, 1992). The rate of decomposition and the balance between N mineralization and immobilization is also influenced by the initial chemical composition of straw (Swift et al., 1979; Melillo et al., 1982).

Although the effects of straw quality and placement on decomposition are well known (Reinertsen et al., 1984; Christensen, 1986; Beare et al., 1993), few studies have investigated how time-of-incorporation (Christensen, 1985) and irrigation (Schomberg and Steiner, 1999) influence crop-residue decomposition and nutrient release. Fungal and bacterial components of the soil microbial community frequently play very different roles in straw decay and nutrient release (Nakas and Klein, 1980; Holland and Coleman, 1987; Beare, 1997). Their contributions to the size and activity of the soil microbial biomass are often influenced by the environmental conditions (i.e., temperature, moisture, nutrient availability) imposed by different residue management practices (Parr and Papendick, 1978; Doran, 1980; Hendrix et al., 1986; Beare et al., 1992). As a result, the type of residue management employed may be an important factor in determining the nature and extent of organic matter dynamics and nutrient cycling in agricultural soils (Doran, 1980; Holland and Coleman, 1987; Beare, 1997). While there has been considerable research into the effects of straw incorporation on soil N availability, there have been relatively few attempts to relate residue decomposition and microbial activity to patterns of soil N availability and crop production in integrated crop residue management systems.

The objective of this study was to determine the effects of time-of-incorporation and irrigation on barley straw decomposition and microbial activity in relation to soil N availability and crop production over a complete cropping cycle.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description and Trial Design
A field trial was established on a Wakanui silt loam soil (Udic Dystochrept, USDA Soil Taxonomy) at the AgResearch Farm (43°38' S lat., 172°30' E long.) near Lincoln, Canterbury, New Zealand. The soil (0–15 cm) at this site had a pH of 5.3 and contained 31 g kg-1 total C, 3.3 g kg-1 total N, and 20.0 mg kg-1 Olsen P. The site was maintained as a perennial ryegrass (Lolium L.)/white clover (Trifolium angustifolium L.) pasture for 4 yr followed by 2 yr of cultivation and cropping leading up to this study.

The field trial was composed of two time-of-incorporation treatments crossed with two irrigation treatments yielding the following four treatment combinations: (i) AI–Irr, autumn-incorporated residues, summer irrigated; (ii) SI–Irr, spring-incorporated residues, summer irrigated; (iii) AI–NIrr, autumn-incorporated residues, no irrigation; (iv) SI–NIrr, spring-incorporated residues, no irrigation.

Treatments were laid out in a randomized block design with each treatment replicated six times, giving a total of 24 plots. Plots (10 by 15 m) were cropped to barley ("Nugget") in the year prior to this study (1996-97). After harvest of the grain in March 1997, barley residues (~7 Mg ha-1) were thrashed and left on the soil surface. The time-of-incorporation treatments were initiated in early autumn (May) 1997. Soils in the AI treatments were ploughed to a nominal depth of 15 cm on 2 May, resulting in the burial of most barley residues to a depth of ~15 cm in the soil profile. Barley residues in the SI treatments remained on the soil surface during the winter-fallow period and were incorporated at the time of secondary cultivation in spring. All plots were ploughed, harrowed and rolled, and maxi-tilled (0–15 cm) on 20 through 21 October, prior to sowing the barley crop on 22 October. Recommended rates of fungicides and herbicides (Glean, Versatill, Cereous, Pirimor) were applied pre or postgermination as required. All plots were fertilized at post emergence with 40 kg N ha-1 in the form of Cropmaster 20 (Ravensdown Fertilizer Coop, Christchurch, New Zealand) (20% N, 10% P, 12.5% S).

Time-of-incorporation treatments were imposed on irrigated and nonirrigated plots that had been established ~18 mo prior to the commencement of this study. During the current study, irrigated plots received water at weekly intervals during the summer growing season, from 27 Nov. 1997 to 8 Feb. 1998. Irrigation rates were calculated using a water budget method to maintain the soil moisture between field capacity and a 20-mm soil water deficit in the top 20 cm of the soil profile. Mean annual rainfall at this site is 680 mm. Approximately 30% of the annual rainfall occurs during the winter months (~70 mm mo-1, June–August), the balance being distributed relatively evenly over the remainder of the year (~52 mm mo-1, September–April). Long-term records show that precipitation exceeds evapotranspiration from April to August with soil drainage likely to occur from July to the end of August (Francis et al., 1992). Daily meteorological data were recorded at a station ~1 km from the trial site during the study period (Fig. 1A) . In this study, total rainfall was 335 mm during the winter-fallow phase (May–October 1997) and 116 mm during summer cropping phase (November 1997–January 1998). This distribution of rainfall was well within the normal range for this site. During the summer cropping phase, soil water deficits in nonirrigated soils increased from ~20 mm in early September prior to sowing the barley crop to ~500 mm at crop harvest and never dropped below ~200 mm after canopy closure (220 d from start of field trial in May) (Fig. 1B).



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  		Fig. 1. (A) Rainfall (bars) and surface (0–10 cm) soil temperature (line) between 1 May 1997 and 16 Feb. 1998 and (B) the predicted soil moisture deficit (mm) in the irrigated and nonirrigated (rainfed) soils (0–20 cm) during the period of summer irrigation (3 Nov. 1997–13 Feb. 1998).

 
Straw Decomposition and Nitrogen Release
Barley straw decomposition, microbial activity, and N dynamics were studied using a litterbag decomposition technique (Beare et al., 1992). Litterbags (20 by 20 cm, 4-mm mesh) were filled with 30 g of postharvest barley straw which had an initial chemical composition of 4 g N kg-1, 79 g lignin kg-1, and 390 g cellulose kg-1 (As determined by proximate analysis [Goering and van Soest, 1970]). These were buried to a depth of 15 cm at autumn cultivation (AI), or placed on the soil surface over winter and buried at time of secondary cultivation in October (SI), prior to sowing the spring barley crop. The straw application rate was equivalent to 7 Mg ha-1.

Barley straw was collected as standing stubble from research plots adjacent to the trial site in autumn 1997. Litterbags were prepared from fibreglass-nylon material and filled with 30 g of oven-dried (60°C) barley stems and leaves that had been cut to 5-cm lengths. Litterbags were randomly assigned to treatment plots. Eight litterbags were placed in each of the six replicate treatment plots on 5 May, yielding a total of 48 litterbags per treatment. Litterbags were placed horizontally at a depth of 15 cm in the AI treatments and on the soil surface in the SI treatments. Ten additional litterbags were transported to the field, returned to the laboratory and reweighed to correct for small amounts of residue mass loss resulting from transport and handling. All litterbags remaining in the plots following the winter-fallow period were carefully removed (20 October) prior to cultivation and stored in a cold room (4°C). The litterbags were returned to the plots on 23 October where they were buried vertically in the top 15 cm of the soil profile.

Litterbags were collected from the plots at regular intervals from autumn cultivation (15 May 1997) through to harvest of the barley crop (9 Feb. 1998). The straw remaining on each sample date was removed from the litterbag, shaken gently over a sieve (1 mm) to remove the majority of the soil and the total wet weight recorded. Subsamples (2.0 g) were removed and oven-dried at 60°C for 48 h to determine gravimetric water contents and then ground to pass a 0.5-mm sieve. Subsamples (0.5 g) of the ground residues were ashed in a muffle furnace at 550°C for 5 h to determine their ash content. The ash content of the residues was used to adjust their dry weights and N contents to an ash-free dry weight (AFDW) basis to account for variable quantities of contaminating soil. The total N content of the residues was determined by Dumas combustion on a LECO C/N/S analyzer (LECO Corp., St. Joseph, MI) at 1050°C (McGill and Figueirdo, 1993).

Microbial Populations on Straw
The size of the metabolically active microbial biomass on the residues was quantified with a substrate-induced respiration (SIR) method (Anderson and Domsch, 1975) adapted for use on plant residues (Beare et al., 1990). Briefly, a subsample (2–4 g) of field-moist residue from each litterbag was weighed into a conical flask, amended with 3 mL of glucose solution (80 mg glucose g-1 dry wt.) and incubated for 4 h at 23°C. Respired CO2 was trapped in 10 mL of 0.04 M NaOH held in the sidearm of the flask. To determine the quantity of respired CO2, 5 mL of the 10-mL trap was diluted to 0.004 M NaOH and titrated with standardized 0.004 M HCl following the addition of BaCl2 and phenolphthalein indicator. All samples were corrected for the CO2 content of sample blanks.

A fluorescent staining technique was used to quantify populations of total and viable fungi and bacteria (Anderson and Slinger, 1975). With this procedure, viable, nucleic acid-containing bacterial cells and fungal hyphae fluoresce red while nonviable, proteinaceous cells and hyphae fluoresce blue (Johnen, 1978) when viewed under a fluorescence microscope. The method used here was adapted from that of Bloem et al. (1992). Briefly, subsamples (2.0 g) of residues from each litterbag were homogenized in a blender at low speed with 100 mL of sterilized water, and a 10-fold dilution of the suspension was prepared. A 10-µL aliquot of each residue suspension was spread evenly in each of four wells (6-mm diam.) on a glass microscope slide, air-dried, and heat fixed by passing through a flame. Sample wells were amended with a drop of differential fluorescent stain composed of Europium chelate and fluorescent brighter (Anderson and Westmoreland, 1971) and incubated (60 min) at room temperature in a dark, sealed container. Excess stain was washed free in a gentle stream of 50% ethanol (v/v), the slides air-dried, and the sample wells covered in Fluoromount and a glass cover slip. Direct counts of red and blue fluorescing bacterial cells and fungal hyphae were made at x1000 under oil immersion using an Olympus BH2 microscope (Olympus Corp., Lake Success, NY) equipped for epifluorescence microscopy.

Soil Nitrogen Availability
Soil samples were collected on six sampling dates, corresponding to six of the eight litterbag collection dates throughout the study. On each sampling date, a total of five soil cores (2.5-cm diam.) were taken to a depth of 25 cm from each plot. The soil cores were divided into 0- to 5-, 5- to 10-, and 10- to 25-cm depth intervals and composited by depth. Ammonium and NO-3-N were extracted from field moist soil (5 g, <2 mm sieved) with 40 mL of 2 M KCl, and their concentrations determined using standard colorimetric procedures (Keeney and Nelson, 1982). Total soil C and N concentrations were measured on air-dried soils taken from the first (15 May) and last (15 September) sample dates during the fallow phase and the final sample date (9 February) in the cropping phase. The C and N analyses were carried out on a LECO C/N/S analyzer as described above.

Crop Production
Aboveground dry matter (straw) and grain yields were measured at harvest in February 1998. Briefly, total aboveground dry matter was harvested by hand from 2 quadrats (each 0.5 m2) per plot, oven-dried (50°C), the grain heads removed, and the component dry weights recorded. The N content of straw and grain was determined on a LECO C/N/S analyzer as described for straw decomposition. The yields and N concentrations of straw and grain measured in this study were well within the normal range for irrigated and nonirrigated barley crops at this site.

Statistical Analysis
The data were examined using analysis of variance (ANOVA) procedures. Separate analyses were carried out for each sample date and depth where appropriate. Significant differences among treatment means were assessed by least significant difference tests (LSD, P < 0.05). The relationship between straw mass loss and SIR was investigated with a simple linear regression. All analyses were carried out using Genstat 5 (Genstat 5 Committee, 1997).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Straw Decomposition
Straw placement (surface vs. buried) had a significant effect on the decomposition of barley straw during the winter-fallow phase (May–September) of this study. At the time of spring cultivation, the weight loss of buried straw from the AI treatments averaged 33% as compared with 18% for surface straw from the SI treatments (Fig. 2) . The mass loss from surface and buried straw during the winter-fallow phase followed a linear trend of decomposition. Barley straw decaying on the soil surface lost ~0.15% of its initial mass each day during this period. By comparison, incorporated straw lost ~0.26% d-1. This linear pattern of decomposition is consistent with other studies of cereal straw decomposition carried out under cool, moist climatic conditions. In a study from Denmark, Christensen (1986) reported weight loss rates of 0.07% d-1 for incorporated barley straw during the cool season (November–May). Douglas et al. (1980) also reported linear rates of decay for surface and buried autumn applied wheat straw, however, the mass loss from buried straw slowed temporarily during periods when the soil temperature fell below 4.5°C. Similarly, a previous study at this site (Cookson et al., 1998) showed that the mass loss rate of AI barley straw slowed substantially during the peak winter months of July and August when mean daily temperatures averaged <6°C.



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  		Fig. 2. Effects of time-of-incorporation and irrigation on the percentage of initial barley straw remaining over 320 d of decay. AI = autumn incorporated, SI = spring incorporated, Irr = summer irrigated, NIrr = nonirrigated. Values are means (n = 6) of mass remaining data expressed on an ash-free dry weight (AFDW) basis. Error bars are least significant differences (LSD, P = 0.05, d.f. = 15) for within sample date comparisons.

 
By harvest, nearly all of the difference in mass loss between AI and SI straw (17%) from Nirr treatments could be attributed to decomposition in the fallow phase (Fig. 3) . Although Nirr, SI straw continued to lose mass at a relatively linear rate (0.17% d-1) during the cropping phase, the decomposition of AI straw was reduced to a very slow rate of decay (0.05% d-1) during the dry summer months (December–February). This difference in mass loss is probably attributable to differences in the chemical composition of the surface and buried straw at the time of spring incorporation that are a product of their winter decay.



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  		Fig. 3. Effects of time-of-incorporation and irrigation on the total mass loss of barley straw during the fallow and cropping phase. Irr = summer irrigated, NIrr = nonirrigated. Values are means (n = 6) of mass loss data expressed on an ash-free dry weight (AFDW) basis. The LSD bar (P = 0.05, d.f. = 15) is for comparisons of total mass loss (Fallow + cropping phase).

 
There was no apparent residual effect of irrigation in the previous year on the decomposition of barley straw during the fallow phase (Fig. 2). The extent to which irrigation affected barley-straw decomposition during the cropping phase (October–February) depended on time-of-incorporation. Irrigation increased straw decomposition during the cropping phase by 68% in AI treatments compared with only 37% in SI treatments (Fig. 3). By harvest time, AI and SI straw of the Irr treatments had lost 79 and 55% of their initial mass, respectively. Schomberg et al. (1994) also reported that irrigation had a smaller effect on the decomposition of surface-applied wheat (Triticum aestivum L.) and sorghum [Sorghum bicolor (L.) Moench] straw than on buried straw. However, in their semi-arid agriculture system, irrigation and straw placement treatments were initiated in the spring and imposed throughout a 12-mo study period. The authors argued that this difference in decomposition was largely because of a loss of soluble components leached from surface straw during irrigation events. Other research has shown that cold water extraction of soluble components from wheat (Reinertsen et al., 1984) and barley (Christensen, 1985) straw can dramatically reduce their rate of decomposition.

Microbial Populations and Activity
In this study, we did not quantify the effects of winter placement on the water-soluble components of barley straw remaining at the time of spring incorporation and the start of the cropping phase. However, it is possible that the leaching of water-soluble components from surface-applied barley straw during winter precipitation events contributed to differences in populations of bacteria and fungi on surface and buried straw at the end of the winter-fallow phase (Table 1). On the September sample date, populations of viable, nucleic acid-containing bacteria (Europium chelate stained) and lengths of total fungal hyphae were significantly higher on buried AI than surface-placed straw. The populations of residue-borne bacteria and fungi reported here were similar to those reported from a previous study of barley-straw decomposition at this site (Cookson et al., 1998).


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  		Table 1. Effects of residue management practices on populations of viable bacteria (x 109 g-1 AFDW) and lengths of total fungal hyphae (m g-1 AFDW) on decaying barley residues collected at the end of the fallow phase (15 September) and the end of the cropping phase (9 February).

 
Placement of straw appeared to have a much greater influence on the size of residue-borne bacterial populations than fungal populations at the end of the winter-fallow phase (Table 1). In September, the population of bacteria on buried (AI) barley straw was 2.9 times higher than that of surface straw (AI). In contrast, the total fungal population on buried straw was only 1.26 times higher than that of surface straw. This difference is consistent with the hypothesis that fungi make up a relatively greater proportion of the microbial population on surface placed crop residues than on buried residues (Holland and Coleman, 1987), such as has been shown for decaying winter rye (Secale cereale L.) straw in conventional and no-tillage systems in Georgia (Beare et al., 1992). Unlike bacteria, fungal hyphae are not restricted to water films and can maintain growth and activity during periods of intermittent wetting and drying owing to their greater tolerance to low water potentials (Griffin, 1981).

In this study, the effect of winter straw placement on the decomposition response of barley straw to summer irrigation appeared to be related to the size of the residue-borne microbial populations at the start of the cropping phase. The population of bacteria on SI straw from the irrigated treatment increased more than four-fold between the end of the fallow phase and the end of the cropping phase. However, their population on the final sample date (February) remained significantly lower than that of AI straw, which had a much larger population of bacteria at the time of spring incorporation.

Substrate-induced respiration measures the size of the potentially active microbial biomass rather than the specific activity of the biomass at time of sampling. Overall, SIR rates remained relatively low during the winter-fallow period, ranging from 180 to 420 µg CO2-C g-1 AFDW h-1 (Fig. 4) . Although these rates are low by comparison to irrigated straw during the cropping phase, they were very similar to those reported in another study of winter barley-straw decomposition from this site (Cookson et al., 1998). By the end of the fallow phase, SIR rates on buried straw were, on average, 56% higher than those of surface straw, indicating that buried straw supported a larger population of heterotrophically active microorganisms than surface straw. This difference is consistent with the direct counts of bacteria and fungi and the faster rate of mass loss from buried straw as compared with surface straw during this period.



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  		Fig. 4. Effects of time-of-incorporation and irrigation on substrate-induced respiration (SIR) rates from barley straw over 320 d of decay. AI = autumn incorporated, SI = spring incorporated, Irr = summer irrigated, NIrr = nonirrigated. Symbols represent means (n = 6) of SIR expressed on an ash-free dry weight (AFDW) basis. Error bars are least significant differences LSD (P = 0.05, d.f. = 15) for within date comparisons.

 
The size of the active biomass on nonirrigated straw remained relatively constant during the cropping phase. Substrate-induced respiration rates on AI straw were on average ~70% higher than those on SI straw during the summer period, though the difference between nonirrigated treatments were only marginally significant. Given the relatively large increase in fungal and bacterial populations on nonirrigated straw between the end of the fallow phase and the end of the cropping phase, the SIR results suggest that the potentially active biomass declined in proportion to the total microbial biomass by the end of the cropping phase. It is probable that the metabolic activity of the microbial biomass on surface residues varied temporarily in response to more extreme changes in environmental conditions (Neely et al., 1991).

The effect of irrigation on the size of the active biomass during the cropping phase depended on the placement of straw during the winter-fallow period. Substrate-induced respiration rates on AI straw increased 219% between the September and February sample dates, while those of the SI straw showed a 153% increase. Because microbial activity is directly responsible for much of the mass loss from decomposing plant residues in most agricultural systems, the size of the metabolically active biomass should be related to the rate of mass loss. Indeed, previous research has shown that the overall average SIR rate on decaying plant residues is positively related to their rate of decomposition (Neely et al., 1991; Schomberg and Steiner, 1997). We have not attempted to relate SIR rates to decomposition coefficients in this study because of the biphasic nature of the treatments imposed during the trial. However, our results did show a positive linear relationship between the overall average rate of SIR and the total mass loss from barley straw by the end of the cropping phase (Fig. 5) . Further research is needed to determine whether a one-off measure of SIR, coupled with information on temperature and moisture, may be used to predict straw decomposition rates in the field.



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  		Fig. 5. The relationship between overall average SIR rates and the total mass loss of barley straw at harvest. The symbols represent values from individual treatment plots. The solid line is the linear regression of straw mass on SIR (equation and r2 are shown).

 
Nitrogen Availability
Owing to the high initial C/N ratio (106:1) of barley straw, relatively little N was released from decaying straw over the course of this study (Fig. 6) . By the end of the fallow phase, litterbags had lost 10 to 25% of their initial N content. Based on a residue return rate of ~7 Mg ha-1, the release of N from barley straw during the fallow period was equivalent to ~3 to 6 kg N ha-1. At harvest, AI straw from the irrigated treatments had lost 19% of its initial N (~5 kg N ha-1), while straw from the other three treatments retained almost all of the original N. These contributions of N from the decomposition of straw are of little agronomic significance.



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  		Fig. 6. Effects of time-of-incorporation and irrigation on the N remaining in barley straw over 320 d of decay.AI = autumn incorporated, SI = spring incorporated, Irr = summer irrigated, NIrr = nonirrigated. Symbols represent means (n = 6) of SIR expressed on an ash-free dry weight (AFDW) basis. Error bars are least significant differences (LSD) (P = 0.05, d.f. = 15) for within date comparisons.

 
While the effects of time-of-incorporation and irrigation on barley straw N dynamics were small, there were relatively large effects of the treatments on soil mineral N content (Fig. 7) . Cultivation and incorporation of barley straw in the autumn (AI) resulted in significantly greater quantities of mineral N in the topsoil (0–25 cm) during the fallow period, as compared with plots that were not cultivated and where residues remained on the soil surface (SI). Over the winter period, 83 to 94% of the mineral N was in the form of NO3-N. In September, 1 mo prior to spring cultivation and sowing, autumn cultivated treatments had 69 to 76 kg N ha-1 in the top 25 cm, which was ~28 kg more N than was recovered from the uncultivated SI treatments. Most of the additional N in the AI treatments was recovered from the 10- to 25-cm sample depth where it was susceptible to loss through leaching or denitrification during the spring period while crop productivity was still low. These findings suggests that the mineralization of N that results from autumn cultivation far exceeds any N immobilization that may result from incorporation of a high C/N ratio straw such as was used in this study. Francis et al. (1992) also reported effects of winter cultivation timing on soil mineral N levels in an arable cropping soil at this site. In their study, May-cultivated soils had topsoil (0–25 cm) mineral N levels (60–75 kg N ha-1) on 17 September that were remarkably similar to those reported in this study.



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  		Fig. 7. Effects of time-of-incorporation and irrigation on topsoil mineral N levels at three depths during the 320-d study period. AI = autumn incorporated, SI = spring incorporated, Irr = summer irrigated, NIrr = nonirrigated. Bars represent means (n = 6) of total mineral N partitioned by sample depth. Error bars are LSD (P = 0.05, d.f. = 15) for within date comparisons of total mineral N. On all sample dates, the treatments follow the same order as given for 15 May.

 
In November, shortly after spring cultivation and sowing of the barley crop (but prior to initiating the irrigation treatments), mineral N levels were uniformly high in all treatments, ranging from 114 to 134 kg N ha-1 in the top 25 cm (Fig. 7). In the AI treatments, most (82%) of the increase in mineral N from September to November could be accounted for by the addition of fertilizer N (40 kg N ha-1) at sowing. However, in the SI treatments, 50% (39 kg N ha-1) of the mineral N increase could be attributed to net N mineralization following cultivation and incorporation of straw in October. Of the total topsoil N load in November, 37 to 39% was located in the surface soil (0–5 cm) and 38 to 45% was located nearer the bottom (10–25 cm) of the plough layer. Soil mineral N levels declined substantially in all treatments during the summer cropping phase (Fig. 7). At the time of crop harvest in February, topsoil in the nonirrigated treatments retained ~44 kg N ha-1 which was ~71 kg N ha-1 less than was recovered from the same depth in November. In the irrigated treatments, soil mineral N levels at harvest averaged 24 kg N ha-1 which was about 105 kg N ha-1 less than the N load in November. In December, 45 to 57% of the topsoil (0–25 cm) N was recovered from the surface soil (0–5 cm), however, by harvest (February) only 25 to 30% of the topsoil N was held in the surface soil. This result suggests that a relatively greater proportion of the mineral N uptake by irrigated barley was from the top 5 cm of the soil profile.

There was a highly significant effect of irrigation on topsoil mineral N levels during the cropping phase (Fig. 7). In December, ~2 wk after initiating the irrigation treatments, topsoil mineral N levels were 45 to 68% higher in the nonirrigated than in the irrigated treatments, amounting to a difference of 22 to 30 kg N ha-1. Similar effects of irrigation on soil mineral N levels were recorded for the final sample date at crop harvest. Effects of the treatments on topsoil mineral N levels are a product of several dynamic processes and must, therefore, be interpreted with caution. In addition to the effects of cultivation, irrigation is likely to enhance N mineralization, particularly where soil water deficits are otherwise high. However, irrigation is also likely to increase crop production and, therefore, mineral N uptake by the crop. Where mineral N availability exceeds the demand of the crop and immobilization by the microbial biomass is low, NO3-N may be lost through denitrification or leaching from the root zone.

Crop Production
Irrigation also had a significant effect on the dry matter yield of barley straw and grain (Table 2). On average, barley plants from the irrigated treatments produced 50% more straw and 57% more grain than plants from the nonirrigated treatments. Although there were no significant effects of incorporation time on the yields of nonirrigated crops, where irrigation was used, cultivation and incorporation of straw in the autumn (AI) resulted in significantly greater quantities of barley straw and grain than in SI treatments. Nitrogen concentration in the crop was also affected by the management treatments. As expected, the N concentration in straw and grain of irrigated barley crops was much lower (58 and 71%, respectively) than that of nonirrigated crops. In spite of their higher dry matter yields, straw and grain from the AI treatments had higher concentrations of N than the crops of the SI treatments.


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  		Table 2. Effects of residue management treatments on yield and N concentration of barley straw and grain at harvest.{ddagger}

 
The differences in barley dry matter yields and N concentrations among the treatments translated into significant differences in N uptake by the crop (Table 3). Overall, cultivation and incorporation of the straw in autumn resulted in a greater uptake of N in barley straw over that of the SI treatments. The differences between treatments were more pronounced for the irrigated crops. Overall, the effect of irrigation on total straw N was only marginally significant (P = 0.054). In contrast to straw, there was a highly significant interaction between time-of-incorporation and irrigation on the amount of total grain N. For crops under irrigation, total grain N was higher where the soils were cultivated and the straw incorporated in the autumn as compared with the spring. On average, there was an additional 26 kg N ha-1 in the grain from autumn cultivated as compared with spring cultivated plots under irrigation. However, there were no differences in the amount of grain N recovered from nonirrigated crops.


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  		Table 3. Effects of residue management treatments on the amount of N taken up by the crop.

 
Francis et al. (1992) also reported effects of winter cultivation timing on the yield and N content of grain and straw from a spring-sown wheat crop. However, in their study, the yield and N content of wheat straw and grain were marginally greater where the primary cultivation occurred in mid winter (July) as opposed to early autumn (March) but neither were different from the late spring (October) ploughed treatments. These differences were attributed to greater N leaching losses from early autumn (March) than mid-winter (July) cultivated treatments. A key difference between our study and that of Francis et al. (1992) is that the latter involved ploughing-in of a 3-yr-old leguminous pasture, whereas in this study the site had been cultivated and cropped for 3 yr prior to initiating the trial.

Overall, there was a highly significant effect of incorporation time on total N in the barley crop. On average, barley plants from the AI treatments retained 18 kg ha-1 more N than was recovered from the barley crop in the SI treatments. Although the effects of irrigation were only marginally significant (P = 0.10), there was a strong interaction between time-of-incorporation and irrigation. Under irrigation, the N uptake by barley plants from the AI treatments was ~40% greater than in the SI treatments. There were no differences in the N uptake by barley from nonirrigated treatments.

In general, the effects of irrigation on crop production were consistent with the changes in soil mineral N levels during the cropping phase of this study. In nonirrigated plots, much of the N held in the barley crop at harvest could be attributed to the decline in mineral N (71 kg N ha-1) during the cropping phase. This perceived decline in mineral N does not, of course, account for any additional mineralization or losses of N that may have occurred during the cropping phase. The crop yields and N uptake of irrigated crops were only partially explained by differences in mineral N during the cropping phase. Indeed, crop residue management during the winter-fallow period had a significant influence on barley crop production. The barley crop in AI treatments may have benefited from the additional mineral N in the topsoil at sowing. Alternatively, cultivation and incorporation of straw in the spring may have increased immobilization of N in the soil microbial biomass and slowed N turnover, resulting in a reduced supply of mineral N to the crop. While the benefits of autumn cultivation and incorporation of straw are relatively clear, the mechanisms that account for the increase in barley crop production require further investigation.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The results of this study on a temperate, winter-moist soil indicate that management of postharvest crop residues during the autumn-winter period can be important for determining the pattern and rate of residue decomposition, the release of soil mineral N and the performance of spring sown cereal crops. Under nonirrigated conditions, most of the difference in the decomposition of barley straw at the time of crop harvest could be attributed to the placement (surface vs. incorporated) of straw during the autumn-winter-fallow period. Contrary to previous reports, incorporation of barley straw in autumn did not result in significant net immobilization or mineralization of residue N in this study. However, autumn cultivation did significantly increase the mineral N content of the topsoil (0–25 cm) during the winter period compared with treatments that remained uncultivated until spring (SI treatments). Most of the additional N (28 kg N ha-1) in autumn cultivated soil was recovered from 10- to 25-cm sample depth where it was susceptible to loss through leaching and denitrification during the winter-spring period.

The results of this study also demonstrate that the effects of irrigation on crop residue decomposition and crop performance depend on the time of cultivation and residue incorporation. During the spring-summer cropping period, irrigation increased the decomposition of autumn-incorporated straw by 68% as compared with only 37% for spring-incorporated straw. This effect appears to be related to the size of the residue-borne microbial populations at the start of the spring-summer cropping period. Not surprisingly, irrigation resulted in a large increase in barley straw dry matter and grain yield relative to nonirrigated (rain-fed) crops. However, this study also demonstrates that autumn-cultivation and incorporation of cereal crop residues can significantly enhance the effect of irrigation on the performance of a spring-sown crop as compared with soils where the residues are incorporated in spring. The grain yield and N content of irrigated spring sown barley from the autumn cultivated treatment was 17 and 36% higher, respectively, than the spring cultivated treatment. While some of this effect may be explained by differences in soil mineral N during the cropping phase, further research is needed to define the specific mechanisms that account for improved crop performance in soils where the residues are incorporated through autumn cultivation.


   ACKNOWLEDGMENTS
 
We are especially grateful to Jacqueline Piercy, Heather Russell, and Richard Gillespie for their assistance in the establishment and maintenance of the field trial and for laboratory analyses. Thanks also to Charles Wright for technical assistance. Prue Williams, Glyn Francis, and Denis Curtin provided suggestions for and critiques of our research. Research was supported by funding from the New Zealand Foundation for Research, Science and Technology.

Received for publication March 29, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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