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Department of Crop and Soil Sciences, Michigan State Univ., W.K. Kellogg Biological Station, East Lansing, MI 48824-1325
* Corresponding author (snapp{at}msu.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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1 Mg ha–1) cover crop systems with a split-plot design of 0 or 5.6 Mg ha–1 poultry manure. A 75-L container experiment used field trial soil to evaluate N dynamics in the presence and absence of N fertilizer. Organic N source (manure, cover crop residues, or both) availability was used to adjust fertilizer rate downward to provide an estimated 224 kg N ha–1 for all treatments. Plant growth, N uptake, and tuber yield were monitored, along with soil organic N status and light-fraction organic matter. In the field, the integrated treatment (179 kg N ha–1 fertilizer + manure) consistently increased tuber yield and N uptake efficiency by 20% compared with the unamended conventional management (224 kg N ha–1 fertilizer). Similarly, tuber yield and N uptake in the integrated treatments of the container experiment were 14 to 33% higher than the fertilized, unamended treatment. In the absence of fertilizer, rye cover crop and manure enhanced tuber yield 40 to 210% compared with unamended plants. The release of N from diverse sources was in apparent synchrony with plant demand, as indicated by monitoring of NO3–N dynamics and the presence of light-fraction N. Although manure application was associated with higher N input, subsoil NO3–N in manured and unmanured treatments averaged 6.7 and 7.9 mg kg–1, respectively. High productivity and N efficiency were associated with integrating organic and inorganic N sources, which represents an environmentally and agronomically sound management strategy.
Abbreviations: LFOM, light-fraction organic matter • NMP nitrogen mineralization potential
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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An ongoing challenge faced by producers is to predict the N mineralization rate from organic sources relative to crop N demand (Wagger et al., 1985; Kumar et al., 2006). Seasonal variation in N availability from manure may be high, as the rate of N release is related to external factors such as temperature and moisture as well as inherent biochemical properties of the manure (Eghball, 2000; Griffin and Honeycutt, 2000; Hartz et al., 2000). Nitrogen release depends on many interacting factors, such as temperature, moisture, and type of residue, including physical and chemical properties (Abdallahi and N'Dayegamiye, 2000; Bending and Turner, 1999; Ranells and Wagger, 1997).
Results from simulation models indicate that the dynamics of N mineralization from manures and N uptake by crops may be highly variable, which in some cases would result in synchrony of N mineralization and plant uptake, and in other cases generates N uptake curves that do not match with N supply (Pang and Letey, 2000). A simulation study by Snapp and Fortuna (2003) that was field tested at one site indicated that potato N uptake and mineralization of N from organic sources could be synchronized if a mixture of residue quality was used, including low-N (wide C/N ratio) and high-N (narrow C/N ratio) tissues. Notably, seasonal temperature and soil temperature had minimal effects on N mineralization relative to residue quality. The limited correspondence between observed and simulated data in some studies indicates the need for additional in-depth research of the processes involved in N mineralization (Henriksen and Breland, 1999).
To optimize the use of organic sources in cropping systems, and to achieve maximum crop yields, combining mineral fertilizer with organic sources (cover crop residues and manure) may be an effective approach to enhance N supply in relation to crop demand (Sikora and Enkiri, 2000), although the presence of certain cover crops has been associated with slow N release and reduced potato yields in some field studies (Griffin and Hesterman, 1991). Perennial ryegrass (Lolium perenne L.) managed as a cover crop combined with fertilizer enhanced crop yields in one study (Torstensson and Aronsson, 2000), whereas cereal rye combined with fertilizer was associated with variable yield reductions in some long-term studies, from nil to severe depending on the year (Johnson et al., 1998; Kuo and Jellum, 2000). The negative effects of a cover crop on subsequent cash crops may be due to complex interactions, including N immobilization (Kuo and Jellum, 2000), depletion of soil moisture (Wyland et al., 1996), or release of allelopathic compounds (Reberg-Horton et al., 2005). A net benefit analysis estimated that a minimum yield increase of 3 Mg ha–1 in U.S. no. 1 tubers would be necessary to support farmer adoption of organic inputs among Michigan potato producers (Labarta et al., 2002).
The objectives of this study were to: (i) evaluate potato yield response to integrated management of organic and inorganic N sources; (ii) evaluate NO3 release and plant N uptake under integrated N management; and (iii) quantify N leaching potential in a potato rotation in the presence of cover crops, poultry manure, and fertilizer on a coarse-textured soil.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Rye was broadcast planted in October of all study years (Table 1). Just before the crop was killed in May, two 0.25-m2 subsamples of aboveground cover crop tissues were taken per plot to determine their dry matter and total N content. The rye was vegetative in all 3 yr of the experiment when it was disk incorporated 1 wk before potato planting (Table 1). Manure was applied using a small broadcast spreader 3 d before incorporation of the cover crop.
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45 kg N ha–1 during the growing season (see below), while a previous study indicated that a rye cover crop would mineralize 10 kg N ha–1 during the same period (Snapp and Borden, 2005).
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56 g) were planted in May of each year at a spacing of 30.5 cm within rows and 86 cm between rows in six-row plots, 17 m in length (Table 1). Potassium as K2O was applied before planting at the rate of 201.6 kg ha–1, following Michigan State University recommendations. Fertilizer N was applied at planting with the seed pieces using a two-row planter with a starter fertilizer that contained P at a rate of 37.6 kg ha–1, in the form of P2O5 in a 7:3 ratio of polyphosphate and orthophosphate. The remaining N fertilizer was applied through multiple splits at hilling and tuberization (Table 1), following standard Upper Midwest potato production practices devised to optimize fertilizer efficiency (Snapp et al., 2002). Tubers were harvested from the center of the plot using a two-row harvester. The tubers were weighed according to size class to determine the total, U.S. no. 1, and oversize tuber yields (Long et al., 2004). Just before vine kill (Table 1), potato plant N content was determined by destructively sampling six plants per subplot and collecting aboveground biomass and associated tubers. These plant tissues were dried and finely ground (<1 mm) before Kjeldahl digestion for N analysis (Bremner and Mulvaney, 1982).
Cover Crop Tissue Characterization
Cover crop tissues were dried at 70°C until no change in weight was detected (4 d). Tissues were weighed to determine dry matter, and finely ground (<1 mm) before Kjeldahl digestion, as described above. Shoot biomass incorporated with the cover crop was 1241 kg ha–1 in 2002, 882 kg ha–1 in 2003, and 1780 kg ha–1 in 2004. This corresponded to 37, 26, and 49 kg ha–1 of N incorporated from cover crop residues in 2002, 2003, and 2004, respectively (N credit used for reducing N fertilizer rate in cover crop treatments was maintained at a uniform 11 kg N ha–1, Table 2). Average weed biomass in the fallowed plots was minimal (<100 kg ha–1 each year); this was determined following the cover crop biomass protocol.
Manure Characterization
Aged poultry manure (chicken layer product, Herbrucks Poultry Ranch, Saranac, MI) was characterized by conducting biochemical analyses on three composite samples of litter taken from bulk storage bags. Elemental (P, K, Ca, and Mg) analysis was performed on dry-ashed samples. Potassium and Ca were analyzed by flame photometry, and P and Mg were determined colorimetrically following standard methodology (Snapp and Borden, 2005). Inorganic N levels for poultry manure were extracted using 1 M KCl and analyzed colorimetrically using ion chromatography (Flow Solution Model IV, OI Analytical, College Station, TX). The manure contained 1243 mg kg–1 P, 6275 mg kg–1 K, 6600 mg kg–1 Ca, and 450 mg kg–1 Mg, and had a pH value (water/manure 1:1 v/v) of 8.9. Organic C was 235 g kg–1, total N was 35 g kg–1, and mineral N content (NH4–N plus NO3–N) was 0.87 g kg–1, representing 2.5% of total N.
For available N determination, a manure incubation method was modified slightly from that described by Hartz et al. (2000), where manure was mixed with sandy soil (1:50 kg kg–1) and moisture content was adjusted to 12%. The sand–manure mixture was incubated at 25°C in open 100-mL containers that were placed in a basin containing 1-cm water depth to maintain a humid environment. The basin was loosely sealed using a transparent plastic sheet to allow gas exchange. Nitrate-N was extracted at 0, 10, 30, 60, 90, and 120 d after the start of the incubation (Nyiraneza, 2003). After 120 d of incubation, NO3–N was 23% of the total N (195 kg N ha–1), for an estimated 44.8 kg ha–1 available N in 5.6 Mg ha–1 poultry manure for 2002 (Table 2). The N concentration of the poultry manure varied slightly, and the adjusted totals were 48 kg N ha–1 available from manure applied in 2003 and 43 kg N ha–1 in 2004.
Soil Nitrogen
Soil samples were collected on 10 Nov. 2003 and 20 Nov. 2004 from the 0- to 20- and 20- to 50-cm depths, using a 5.7-cm-diameter auger to collect eight subsamples from the topsoil and four from the subsoil. These auger borings were thoroughly mixed to create one composite sample per subplot. Inorganic NH4+–N and NO3–N was extracted with 1 M KCl by shaking 10-g samples at 180 rpm for 30 min and filtering through no. 1 Whatman filter paper (moistened first with 1 M KCl to remove possible NO3–N contamination). Ammonium-N and NO3–N were determined in extracts by colorimetric methods using an autoanalyzer (Lachat Instruments, Milwaukee, WI), as described by Keeney and Nelson (1982). The N mineralization potential (NMP) aerobic assay was conducted with subsamples of soil (10 g) incubated at 60% water holding capacity for 30 d at 25°C. Net N mineralization was calculated using the difference between the inorganic N contents at Day 0 and at the end of the incubation period.
Field Experiment Design and Statistical Analysis
The field experiment was performed during three growing seasons. The experiment had a randomized complete split-plot design with four replications and a plot size of 5.4 by 15.2 m. The main-plot treatment was the cover crop system (cereal rye or fallow), and the subplot treatment was manure application (with or without). Analysis of variance was performed using the MIXED procedure in SAS (SAS Institute, 2002). The crop response and soil N data were analyzed using a fixed year effect split-plot model, with cover crop as the main plot and manure as the subplot. Nitrogen recovery efficiency was calculated following Olesen et al. (2007); plant N uptake in shoot and belowground organs was compared to applied inorganic N (as fertilizer and manure), and the percentage of N recovered was determined. Residuals were examined to evaluate the assumptions of normality and equality of variance. If these assumptions were not met, as in the case of plant N and N efficiency, variables were transformed using log10 and reanalyzed. Means were separated using the LSMEANS post hoc test.
Container Experiment
A large-volume container (75 L, 58.4 cm tall and 48.3 cm in diameter) experiment was conducted in 2002 to closely monitor N release, plant N uptake, and NO3–N leaching potential using a randomized complete block design with a factorial treatment combination (2 fertilizer N rates x 4 management systems) and four replications. The soil used in the experiment was collected from randomly selected locations within the field experiment using a shovel (0–20-cm depth) on 2 May 2002, 9 d following the incorporation of manure and cover crops. Soil was collected from the following treatment plots: (i) winter fallow, (ii) winter rye cover crop, (ii) winter fallow with manure, and (iv) winter rye cover crop with manure.
The soil collected in the field was mixed with perlite on a 50% (v/v) basis to reduce compaction during the study period. Two rates of N fertilizer (0 and 224 kg N ha–1) were tested and applied to the container as NH4NO3 in three equal splits. The 224 kg ha–1 rate was adjusted to account for N credits from the organic amendments, as in the field experiment. Potassium and P fertilizers were incorporated by hand in the container at the same rates used in the field (201.6 and 37.6 kg ha–1, respectively). To avoid Ca deficiency in treatments without manure, Ca (as CaSO4·2H2O) was applied at 112 kg ha–1.
Soils mixed with organic sources in the containers were left to mineralize for 10 d before planting two seed pieces of the potato ‘Atlantic’ per container on 13 May 2002. One week after emergence, the less vigorous of the two plants was removed to gain as much uniformity of plants within the experiment as possible. The plants were watered as required to maintain adequate soil moisture, determined by readings of four soil tensiometers installed in two containers (two at 15 cm and two at 30-cm depth). The crop was harvested on 30 July 2002, 78 d after planting, before complete shoot senescence. Shoots were cut at the soil level, weighed, and cut in small pieces. The soil from each container was sieved (4 mm) to recover tubers and roots. Tubers, roots, and shoots were washed, weighed, and dried at 70°C until no change in weight was detected (4 d). Manure, plant tissues, and soil samples were ground (<1 mm) before analysis. Total C in manure and soil samples was analyzed by the Walkey–Black method (Nelson and Sommers, 1982), while manure and plant N content were determined using micro-Kjeldhal digestion (Bremner and Mulvaney, 1982).
Light Fraction
Light-fraction organic matter (LFOM) was extracted from finely ground (<0.25 mm) soil (10 g) using 50 mL of NaI solution (specific gravity 1.59 g cm–3) (Janzen et al., 1992; N'Dayegamiye and Tran, 2001). The suspension was centrifuged at 2500 rpm for 30 min. The supernatant was filtered under suction through a 0.45-µm filter. The procedure was repeated to facilitate complete removal of the light fraction. The residue (LFOM) remaining on the filter was dried at 105°C and weighed. The N and C content of the light fraction was determined as previously described for the manure.
Nitrate-Nitrogen Dynamics
To monitor soil NO3–N during the growing period, anion exchange resin membranes fixed into plastic probes (Plant Root Simulator, Western Ag Innovations, Saskatoon, SK, Canada) were used. For each container, two membranes were used; one membrane was inserted in the 0- to 15-cm depth to monitor NO3–N release, and the other was inserted in the 40- to 55-cm depth to estimate NO3–N susceptible to leaching. The membranes were attached to wires to facilitate retrieval and minimize soil disturbance at biweekly intervals when the membranes were exchanged. Membrane extraction and preparation followed supplier protocol (www.westernag.ca/innov/protocol.php; verified 11 June 2007). The membranes were charged by being soaked in fresh aliquots of 0.5 M NaHCO3 for four consecutive 1-h intervals while being slowly shaken at 100 rpm. The membranes were rinsed thoroughly with deionized water before installation. Following the burial period and retrieval from the containers, the membranes were cleaned with distilled water to completely remove soil. Each membrane was then placed in a zip bag and soaked in a 25 mL of 0.5 M HCl solution while being shaken at 100 rpm for 1 h. The extract was placed in vials for NO3 analysis by flow injection using Cd reduction and colorimetric detection (Flow Solution IV, OI Analytical, College Station, TX).
Following potato harvest on 30 July 2002, eight subsamples of soil were collected using an auger (5.7 cm) from 0- to 15-, 15- to 30-, and 30- to 45-cm depths. Soil samples corresponding to each soil layer were combined and thoroughly mixed to create composite samples. Soil inorganic N (NH4–N and NO3–N) was extracted using 1 M KCl samples as described for the field experiment. Taking care not to compact the soil, three volumetric soil cores (2 cm) were removed, dried at 70°C, and weighed to calculate bulk density.
Container Experimental Design and Statistical Analysis
A randomized complete block design was used, with two main factors and four replications. A main factor of fertilizer (applied or not applied) was tested in combination with four different soil management methods (fallow, rye cover crop, manure application, and rye cover crop plus manure application). An ANOVA was performed using PROC GLM in SAS (SAS Institute, 2002). Nitrates released in the top and bottom soil layers of each container were monitored every 15 d, and the NO3 data was handled as a repeated measure of the corresponding experimental unit. Residuals were examined to evaluate assumptions of normality and equality of variance. If these assumptions were not met, as in the case of particulate organic matter N, plant nutrients, and N recovered, variables were transformed using log10 and reanalyzed. Means were separated using the LSMEANS post hoc test.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Interestingly, no treatment effect of increased plant N uptake was observed in plots amended with organic fertilizer sources. Plant N uptake in the no-manure fallow, no-manure rye cover crop, fallow plus manure, and rye cover crop plus manure treatments averaged 113.6 kg N kg ha–1 (Table 2). Overall, the 2002 U.S. no. 1 tuber yield was increased by 5.1 Mg ha–1 in manured relative to unmanured treatment plots (Table 3). Since there was no corresponding increase in N uptake, this yield increase was either induced by factors other than N uptake, or was related to the timing of N availability.
Nitrogen uptake efficiency has been observed to decrease at high rates of N application and also depends on factors such as the application method and type of fertilizer used. Tyler et al. (1983) reported potato N efficiency in a California field study to be 57% at N rates up to 200 kg N ha–1, but only 39% at 270 kg N ha–1. In the present field experiment, where the fertilized bare fallow treatment received 224 kg N ha–1, potato N uptake (sum of above- and belowground N accumulation) on 3 Sept. 2002 was nearly 50% (111 kg ha–1) of N fertilizer applied. Similarly, Errebhi et al. (1998) found that N uptake by potato (cv. Russet Burbank) fertilized at a rate of 302 kg N ha–1 recovered 56% of fertilizer N applied if soil conditions did not favor leaching. Our values for N uptake in plants treated with fertilizer and manure (about 115 kg N ha–1) are close to those found by Saffigna et al. (1977) in a Wisconsin fertilizer study where plant shoot and tuber N uptake ranged from 120 to 145 kg N ha–1.
Nitrogen uptake efficiency (NUE) was evaluated as a function of applied inorganic N in our study. Plots receiving manure had moderately increased potato yields (Table 3) and the same level of plant N uptake as unmanured plots (plant N measured in 2002 only; Table 2). Inorganic N applied in the manure plus fertilizer treatment was 181 kg N ha–1, whereas 224 kg ha–1 of inorganic N was applied in the control plots. Thus, the apparent N recovery efficiency (Table 2) was higher from manured (62%) than from unmanured treatments (52%). This is similar to or more efficient than previously reported N recovery in field-grown potato (Errebhi et al., 1998; Tyler et al., 1983). Nitrogen uptake efficiency ranged from 15 to 38% in barley (Hordeum vulgare L.) grown in soil amended with 15N-labeled poultry manure (Thomsen, 2004). Efficiency of N recovery from manure was also variable, 16 to 50%, in a multiple-site cropping systems experiment where no fertilizer was applied (Olesen et al., 2007). Interestingly, in the Olesen study, the highest level of N recovery was observed at a coarse-textured site with a soil type similar to that of our field experiment. It was beyond the scope of our study to consider long-term NUE and the fate of N from organic sources. Monitoring of NMP and light-fraction N did provide insights into intermediate fates for N and NUE in organic-amended systems, as discussed below.
Field Soil Nitrogen
Fall NO3–N in the topsoil (0–20-cm depth) was not influenced by treatment in either year (4.1–7.5 mg kg–1; Table 3). Cover crop presence was associated with a moderate reduction in subsoil (20–50-cm depth) NO3–N, consistent with earlier findings from monitoring of commercial potato rotation systems where cereal cover crops have been adopted (Weinert et al., 2002). At the end of the growing season, NO3–N in soil sampled at the 25- to 50-cm depth in the plots with rye cover crop (6.9 mg kg–1) was less than that measured in the corresponding soil samples from the fallow plots (8.0 mg kg–1) (averaged across 2003 and 2004 data; Table 3). Rye acquisition may have reduced soil NO3–N levels, although fall root growth is not expected to be extensive in an October-planted cover crop. The minimal C inputs in the fallow treatment may have reduced N assimilation into the soil organic matter pool, and increased soil NO3–N levels in the soil profile after harvest.
There was no significant effect of management differences on NMP in our study (Table 3). This result demonstrates the complexity of understanding N assimilation into organic pools and subsequent release in integrated cropping systems (Drinkwater and Snapp, 2007). In a Wisconsin study, manure N applied at three times the rate of fertilizer N was associated with moderate NO3–N levels deep in the soil profile (Munoz et al., 2003). The results of a 5-yr study of catch crop and manure amendment effects on leaching in Swedish row crop systems were consistent with those of our study, as the bare winter system had the highest NO3–N leaching even at moderate fertilizer N rates (Torstensson and Aronsson, 2000). The reseachers concluded that a fall-established ryegrass cover crop was the most important determinant for low NO3–N losses, more so than the rate of fertilizer or manure applied.
Container Experiment
Tuber Yield
Tuber yield varied from 321 to 741 g plant–1 in the zero N fertilizer treatments, and from 766 to 1017 g plant–1 when N fertilizer was applied in combination with organic N sources (Fig. 1
). Potato tuber yield increased markedly with N inputs from organic sources in the unfertilized treatments, from 40 to 210%. Potato tuber yields were highest (P < 0.02) in the organic-amended systems, compared with the fallow treatment, especially where manure was combined with rye (Fig. 1). There was marked consistency between the average tuber yield increase observed in the field (18%) and in the container experiment (20%) when organic sources of N were added, compared with the fallow, inorganic N fertilizer treatment.
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Nutrient Dynamics
In the absence of N fertilizer, nutrient uptake in plant biomass increased 200% or more in the rye plus manure treatment compared with the fallow treatment (Table 4). This was not surprising, as yield and biomass increased twofold in unfertilized treatments amended with organic nutrient sources, relative to the control. Nitrogen fertilizer applied in combination with organic sources did not enhance N uptake compared with N fertilizer applied alone (Table 4), whereas uptake of other nutrients (P, K, Ca, and Mg) was markedly increased in the integrated fertilizer, rye, plus manure treatment. Nutrient uptake was related to the higher potato tuber yields observed in this treatment (Fig. 1). Potassium uptake by potato was increased to the greatest extent with the organic amendments of manure and rye residues; this was expected since the poultry manure supplied a source of K and rye roots may have acquired K from deep in the profile (Table 4). Potato requirements for N, P, and K are high, with complex interactions frequently observed (Parent et al., 1994). Increased plant shoot and root growth associated with increased N supply may explain, in part, the increased uptake observed for other nutrients.
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Investigation of soil NO3–N dynamics in the presence of roots, as monitored using anion exchange membranes, showed that soil mineral N was initially high but decreased with time during plant N uptake (Fig. 2 ). Nitrate depletion during the season due to plant NO3–N uptake was observed also in experiments by Paré et al. (1995) and Jowkin and Schoenau (1998) using anion exchange membranes. Repeated measures ANOVA in our study showed that significant differences observed (P < 0.05) between treatments at the beginning of June largely disappeared by late July (Nyiraneza, 2003). One exception to this trend was significantly increased surface soil NO3–N in the rye cover crop treatment observed in mid-July, a period of mineralization that occurred late relative to the other treatments (Fig. 2). A consistently high level of mineral N was released with time in the rye plus manure treatment compared with other treatments (Fig. 2). This observation is consistent with others who have described the effect of mixed-quality residues (e.g., variable C/N ratio from <10 to >70) on N mineralization (moderate delays of N mineralization with widening C/N ratios of residues) (Malpassi et al., 2000; Wagger et al., 1985). The cumulative NO3–N release (calculated by summing the 15-d release data shown in Fig. 2) was twofold higher in the organic-amended treatments than the unamended soil (Nyiraneza, 2003).
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Soil profile inorganic N (NH4–N plus NO3–N) extracted by 1 M KCl at harvest is presented in Table 5. The topsoil (0–20-cm depth) inorganic N pool in manured soil was larger than that in unmanured soil. There was no difference observed, however, in inorganic N concentration at the 20- to 50-cm depth between manured and unmanured plots (Table 5). These data are generally consistent with the field observations in this study, since the highest organic N input treatment (manure + rye cover crop) was not associated with enhanced soil NO3–N levels in either system (Tables 3 and 5). In contrast to the field experiment, however, a winter cover crop history did not reduce soil NO3–N levels compared with a winter fallow in the container experiment.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication July 17, 2006.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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