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Dep. of Vegetable Crops, One Shields Ave., Univ. of California, Davis, CA 95616 USA
lejackson{at}ucdavis.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Abbreviations: CEC, cation-exchange capacity • DAI, days after incorporation of cover crop residue • IER, ion exchange resin • MBC, microbial biomass C • MBN, microbial biomass N
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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in the groundwater of intensively managed crop systems often exceed the public health drinking water standard due to leaching below crop root zones (Legg and Meisinger, 1982; Howarth et al., 1996; Zhang et al., 1998). Previous work in vegetable crop systems has shown that nonleguminous winter cover crops can assimilate up to 
100 kg N ha-1, thereby reducing the leaching of NO-3–N during winter rainfall periods (Shennan, 1992; Jackson et al., 1993; Creamer et al., 1997; Brandi-Dohrn et al., 1997). Incorporation of cover crop residue into soil temporarily increases microbial activity and consequently alters the amount and seasonality of available inorganic N (Aulakh et al., 1991; Wyland et al., 1996; Kuo et al., 1997; Lundquist et al., 1999). A successful cover crop strategy is to synchronize the release of inorganic N from the cover crop with N demand by the subsequent cash crop so that cover crop-derived N is retained in the system, and N losses via leaching and denitrification of mineralized cover crop-derived N are minimized.
In most cropping systems, cover crops are typically incorporated at a low C/N ratio (i.e., <20) to ensure rapid decomposition and avoid prolonged net microbial immobilization of N that can be deleterious for uptake of N by the subsequent cash crop (Wyland et al., 1995; Ranells and Wagger, 1997a). Mineralization of N from cover crop residue can contribute either a small or a substantial fraction (e.g., 4–30%) of the N assimilated by a subsequent cash crop depending on the attributes of the cover crop, such as C/N ratio, soil type, and management practices (Varco et al., 1989; Harris and Hesterman, 1990; Jensen, 1992; Harris et al., 1994; Ranells and Wagger, 1997b). From a practical standpoint, release of cover crop N should be fast enough to match crop N demand, but not so rapid that NO-3–N losses via leaching and denitrification occur during periods of early season rainfall and preirrigation (Bremer and van Kessel, 1992). This can be difficult to achieve since microbial responses can be very rapid during the first 1 or 2 wk following incorporation of low C/N plant material, at least under nonlimiting moisture conditions, and activity can decline substantially thereafter (Wyland et al., 1996; Lundquist et al., 1999). In soils that are C-limited, microbes may rapidly use the readily available C and N from residue with low C/N ratios, creating a short-lived pulse of activity that cannot be sustained for a long period. By labeling plant material with the stable isotope, 15N, and following the fates of 15N through time to various soil and plant pools, a better understanding of the timing and magnitude of microbial responses, N availability, and N losses to the addition of fresh plant material can be developed.
In this study, such a time course was examined at a site that had been under long-term irrigated vegetable crop production in the Salinas Valley, California, where NO-3–N contamination of groundwater is a serious problem. The soil type, a Chualar loamy sand (fine-loamy, mixed, thermic Typic Argixeroll), has experienced intensive management with high fertilizer and irrigation inputs, frequent tillage, and little return of organic material to the soil; it now has low concentrations of soil organic C and N, as well as low microbial biomass C (MBC) and N (MBN) (Wyland et al., 1996; Table 1) . This offered the opportunity to study cover crop decomposition in a situation where soil microbes undoubtedly experience low C availability during most of the year. The main objectives were to (i) describe the changes in soil responses and 15N fates to addition of a 15N-labeled low C/N cover crop under field management conditions, (ii) determine 15N losses during the first year after cover crop addition, and (iii) assess management implications on the basis of information on the rates of release and availability of cover crop N.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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The soil type was a Chualar loamy sand (fine-loamy, mixed, thermic, Typic Argixeroll) with a high percentage of sand at 0 to 75 cm (Table 1), but with higher silt and clay content in the bottom depth below 75 cm. Water-holding capacity was low. It was higher in the surface (0–15 cm) than in the two middle depths (15–45 and 45–75 cm), but increased in the bottom-most layer. Soil organic C and N were low, even in the surface soil, and decreased with depth except for the lowest depth where clay content was higher. Soil pH and cation-exchange capacity (CEC) were slightly lower in the surface soil than at deeper depths. Soil bulk density was considerably lower in the surface than at the next two lower depths. Environmental data were obtained from a weather station 500 m from the study site (University of California Statewide Integrated Pest Management Project, 1994).
The experimental area was divided into four blocks running perpendicular to the beds, each 16 m long and 4 m wide. The zucchini plants were disked on 6 November 1993, and beds were listed and shaped on 19 November. On 28 February, polyvinyl chloride cylinders were inserted into the soil in two beds. Eight large cylinders per block were pushed into the soil using a backhoe, leaving 3 cm above the soil surface. The cylinders were 25.4 cm in diameter and 60 cm deep, with a sharpened bottom edge that facilitated installation and helped to prevent soil compaction. Ion exchange resin bags (Wyland and Jackson, 1993), 23 cm2 and containing 10 g of AG 1-X8 anion exchange resin, were inserted beneath three cylinders in each block to monitor NO-3–N leaching. Based on previous studies (Wyland and Jackson, 1993), the maximum capacity of these IER bags to adsorb NO-3–N was 16 g m-2. One cylinder in each block contained tensiometers at 15 and 45 cm, which were monitored weekly during the first 4 mo after incorporation.
Phacelia was planted in pots in a greenhouse in late December 1993 and watered with a dilute Hoagland's solution containing 15(NH4)2SO4 and K15NO3 continuously from planting until harvest. The plants were removed from their pots on 18 March. The soil was washed from the roots, and the plant material was divided into shoots, clean roots, and roots slightly contaminated with organic matter fragments. The biomass was divided into equal portions, with each cylinder receiving an amount equivalent to 360.3 g dry shoots m-2, 97.6 g dry clean roots m-2, and 20.5 g dry contaminated roots m-2. Analysis of N, 15N, and C in plant material was with a Europa Scientific ANCA-MS (Europa Scientific, Crewe, UK) at the Department of Soil Science, University of California at Berkeley. Shoots, clean roots, and contaminated roots contained 14.3, 11.5, and 8.2 atom % 15N, respectively, so that each cylinder received a total of 77.6 mg 15N (i.e., 1.53 g 15N m-2 or 11.37 g N m-2) from the cover crop residue. The C/N ratio of the cover crop material was 19:1.
A soil sample was collected from each block prior to residue incorporation, and background 15N levels were determined at all depths. Actual mowing and disking practices were simulated while incorporating the residue. The residue was chopped into 4- to 8-cm pieces and mixed into the 0- to 15-cm soil layer in each cylinder on 20 March. Tillage was simulated with trowels and knives to incorporate the residue and again 22 d later. One week after residue incorporation, sprinkler irrigation delivered 32 mm of water. Two small rainfall events delivered 10 mm of precipitation shortly thereafter. Throughout the experiment, fertilizers were applied in a manner and rate equivalent to best management practices for commercially grown lettuce. An unlabeled, typical pre-plant fertilizer (356 kg ha-1 of 5-17-17; i.e., 1.78 g N m-2) was banded at 10- to 12-cm depth in the planting line of the cylinder to simulate growers' practices at 32 d after residue incorporation (DAI). During the next 4 d, 25 mm of rainfall occurred. Crisphead lettuce (`Target') was planted in the cylinders at 45 DAI (3 May), and starter fertilizer was added at that time (381 kg ha-1 of 5-20-3-1; i.e., 1.9 g N m-2). Seedlings were thinned to one plant per cylinder. One additional fertilizer application occurred at 85 DAI (238 kg ha-1 of 20-0-0-5; i.e., 4.75 g N m-2). Thus, a total of 8.43 g N m-2 was added as inorganic fertilizer to the crop. During the first crop, sprinklers were used to apply 150 mm of water from planting through the first 5 wk, and another 260 mm through harvest. Rainfall during the crop was only 9 mm, giving 419 mm of total water inputs. The lettuce in the cylinders was harvested at 116 DAI on 14 July. A second crop was planted on 22 August and was managed with the same fertilizer and irrigation practices as the first crop. It was thinned on 15 September and harvested on 9 November. Irrigation input was 340 mm of water and rainfall was 15 mm, giving a total of 355 mm during the second crop. Harvest of the second crop occurred at 234 DAI and 79 d after planting.
Cylinder samples were taken 3, 14, and 28 DAI, at mid-crop and harvest of the first crop (72 and 116 DAI), at mid-crop and harvest of the second crop (179 and 234 DAI), and 1 yr after incorporation, on 27 March 1995. Samples taken on the first five sampling dates were analyzed for inorganic N (NH+4–N and NO-3–N), moisture content, MBC and MBN, and 15N enrichment in NH+4–N and NO-3–N, and MBN. Resin bags were sampled at 28, 72, and 116 DAI. Total soil 15N including undecomposed residue was measured prior to cover crop incorporation and then at 14, 72, 116, 234, and 365 DAI.
One cylinder per block was excavated on each sampling date. The entire soil core was removed in layers (0–15, 15–30, 30–60 cm), and a soil core was obtained from beneath the cylinders (60–75 cm). Each soil layer was weighed, mixed thoroughly, and subsampled immediately for KCl-extractable NH+4–N and NO-3–N (100 g soil in 250 g 2 M KCl; three replicates per layer per cylinder), gravimetric soil moisture, and MBC and MBN (0–15 and 15–30 cm soil layers only, two replicates per layer). Only the upper two layers were sampled for microbial biomass, since previous work showed very low levels below 30 cm. The 0- to 15-cm layer was sieved (2-mm mesh) prior to subsampling for MBC, MBN, and inorganic N so that extraction of C and N from undecomposed plant material would not confound these measurements. Also, it was not initially possible to take subsamples of soil with representative amounts of plant material due to the large size of the residue pieces. Thus, these data represent pools in the soil, and do not include microbial and inorganic pools present on the decomposing residue. An unsieved subsample was retained for determination of total organic soil N and its 15N enrichment. Inorganic N was measured from the KCl extracts of soils and IER bags using a Wescan ammonia analyzer (Alltech Assoc., Inc., Deerfield, IL), with a reduction column for NO-3–N determination (Carlson, 1978, 1986). Microbial biomass was determined using the fumigation–extraction technique of Brookes et al. (1985), a modified Kjeldahl digestion for N content (Wyland et al., 1994), and a dichromate digestion for C content (Vance et al., 1987). The flush of C and N after fumigation was converted to MBC and MBN using the published values of 2.64 (Vance et al., 1987) and 1.86 (Shen et al., 1984), respectively.
To prepare KCl extracts and the Kjeldahl digests of the fumigated and control MBN extracts for 15N analysis, the diffusion technique of Brooks et al. (1989) was used. It was modified for diffusion of Kjeldahl digests by using 10 mL of 10 M NaOH rather than MgO to volatilize NH3. When necessary, a 14N internal standard was added to spike samples that were low in N prior to diffusion. Lettuce shoots and roots harvested from the cylinders were oven-dried at 65°C, weighed, ground to a powder in a ball mill at 200 rpm for 4 h, then subsamples were weighed into tin disks for combustion in a mass spectrometer. Soil samples were prepared for 15N of total soil N (0–30 cm depth) analysis by grinding with a mortar and pestle, sieving (2 mm mesh), then grinding any sieved-out plant residue, and returning it to the sample and mixing well. A 40-g subsample was then ground in a ball mill at 200 rpm for 4 h before subsamples were weighed and wrapped in tin disks for combustion in the mass spectrometer. The 15N analysis was conducted at the University of California, Berkeley, on a mass spectrometer (Automated Nitrogen and Carbon Analyzer, Europa Scientific, Crewe, UK). Percentage recovery of 15N in each N pool of the added 15N in the cover crop was calculated as described in Hauck (1982) and Wyland et al. (1995), using the basic equation:
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Statistical analyses were performed in SAS (SAS Institute, 1985), using analysis of variance and GLM procedures. Differences between means were assessed with the REGW Multiple Range Test. In Table 1, mean comparisons are not shown due to the large numbers of comparisons. For these data, any significant changes noted in the text are at P < 0.05.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Air temperatures gradually increased during the postincorporation period, with mean daily air temperatures of 12, 13, 14, 16, and 16°C in March, April, May, June, and July, respectively (University of California Statewide Integrated Pest Management Project, 1994). Corresponding mean soil temperatures at the 15-cm depth were 13, 18, 19, 22, and 21°C. The 4 mo following incorporation of the plant residue were generally free of clouds and fog.
Soil Microbial Biomass
Microbial biomass was initially very low, but increased markedly in the first 2 wk after cover crop incorporation, followed by a sharp decline (Fig. 1c and 2)
. At the onset of the experiment prior to incorporation of cover crop residue, MBN was 15 mg N kg-1 soil (Table 2)
and MBC was 300 mg C kg-1 soil (data not shown) in the 0- to 15-cm layer of soil. At this time, there were 4 g N m-2 (Fig. 1c) and 85 g C m-2 (Fig. 2) as microbial biomass at the 0- to 30-cm depth. Immediately after incorporation, a rapid and significant increase in MBN occurred in the first 3 d in the surface (0–15 cm) layer (Table 2). A slight decrease in MBN also occurred at 15- to 30-cm depth at this time. Since its 15N enrichment was above ambient levels, some residue was probably incorporated deeper than the intended 15-cm depth. During the subsequent week, MBN continued to increase (Table 2 and Fig. 1c). For the next 2 wk, no significant change occurred in MBN. During the subsequent 6 wk (28–72 DAI), MBN declined to the same level before the incorporation of cover crop residue, and remained at this level until lettuce harvest (116 DAI).
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Microbial biomass C increased following incorporation (Fig. 2), with a pronounced increase between 3 and 14 DAI, but 2 wk later, MBC had decreased to below its pre-incorporation levels and remained constant during the subsequent 3 mo. The C/N ratio of the microbial biomass decreased during this period, starting initially at 20:1, then declining to 16:1 between 1 and 2 wk after incorporation of residue and to 7:1 at 4 wk after incorporation, and thereafter remaining at 12:1 (Fig. 1c and 2).
Soil Nitrogen
In the surface layer (0–15 cm), NO-3–N concentration responded fairly quickly to the simulated tillage and the addition of residue, and 30% was derived from cover crop N during the first 2 to 4 wk (Table 2). At 3 DAI, NO-3–N concentrations had doubled compared with initial values. Only a small but significant fraction was derived from cover crop N at this time, indicating that a pulse of NO-3–N production occurred in the 0- to 15-cm layer initially in response to the soil disturbance rather than the addition of residue. Thereafter, at 14 DAI, the NO-3–N concentration in the surface soil remained high, and one-third of it was derived from cover crop N. During the next week, the concentration of NO-3–N increased, and a high percentage was still derived from residue. By the middle of the crop period (72 DAI), NO-3–N in the surface soil had decreased significantly, and this decline continued through harvest (116 DAI), as did the percentage of NO-3–N derived from the residue.
Nitrate from the surface soil moved to lower depths by 2 wk after incorporation of residue, as indicated by the significant increase between 3 and 14 DAI (Table 2). A small amount was derived from cover crop residue. The first irrigation event of 32 mm occurred at 7 DAI. Concentrations and 15N enrichment of NO-3–N continued to increase at lower depths through the middle of the lettuce crop period. The proportion of NO-3–N derived from residue at the second depth (15–30 cm) increased significantly by the time of planting, and there was a slight but significant enrichment of NO-3–N in depth 3 (30–60 cm) at this time. By harvest, NO-3–N concentrations had decreased at all depths, and 15N enrichment was present at all depths from 0 to 75 cm.
In terms of the total NO-3–N pool in the profile (g N m-2 at 0–60 cm depth), a rapid rise occurred during the first 2 wk after incorporation of plant residue, and remained high from before planting through the middle of the crop period, during which time N fertilizer was applied twice (Fig. 1b). A large decrease occurred in the last month of the crop period when lettuce had the highest N uptake (Table 3) . The amount of NO-3–N derived from cover crop N was highest from 14 to 28 DAI and never exceeded 2.0 g N m-2 (0–60 cm) during the 4-mo period after incorporation of plant residue.
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1 mg kg-1 soil. The NH+4–N concentration and its percentage derived from the residue had declined by 14 DAI, and contribution from cover crop N declined thereafter. Slight 15N enrichment of NH+4–N was measured sporadically in the lower depths through the postincorporation period (Table 2), but it is difficult to interpret these results. The recovery of 15N in the total soil N pool (0–30 cm), which included inorganic N and MBN as well as other organic N and any remaining undecomposed residue, declined steadily at successive sampling dates after incorporation (Fig. 4) . By harvest of the first crop, only 63.5 ± 3.1% of the added 15N was recovered in the total soil N.
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Lettuce plants assimilated 20.7% of the N from the cover crop residue N by harvest at 116 DAI (Fig. 5 and Table 3). Similar 15N enrichment was measured in the roots and the shoots. Cover crop N contributed 9.1% of the lettuce N.
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Nitrogen-15 Fates in Subsequent Seasons
The second crop of lettuce used much less cover crop-derived N than did the first crop (Table 3). Only 5.1 ± 0.6% of the cover crop N was present in this lettuce crop at harvest maturity. This accounted for 4.5% of the N in this lettuce crop. Total growth and N accumulation were lower due to cooler temperatures.
Recovery of 15N derived from the cover crop as total soil N continued to decline during the period between July and November, but at a slower rate than during the initial 4 mo (Fig. 4). However, there was no change between November 1994 and the sample at 1 yr after cover crop incorporation in March 1995; both samples contained 46.1 ± 2.1% of the added 15N. At this time, following the winter fallow, MBC was at the same pre-incorporation values as in the previous year (data not shown).
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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20%) with relatively low soil N loss. Soil organic 15N continued to decline for the next 3 mo, but was of minor importance as a source of N for the second lettuce crop. The sandy soil texture probably offered little protection for decomposing material, and this may have hastened the decomposition rate (Ladd et al., 1995). Dilution from a larger pool of exogenous soil N would have further reduced the recovery of 15N by the second crop (Crozier et al., 1998). Thereafter, no change occurred in soil organic 15N, suggesting that the N remaining from the cover crop residue was no longer readily available for net mineralization.
Microbial Biomass and Nitrogen Mineralization of Plant Residue
Despite the twofold increase in MBC and MBN following cover crop incorporation, the 15N enrichment of the MBN remained very low and relatively constant throughout the postincorporation and subsequent cropping periods. Soil microbial biomass appears to have been relying primarily upon soil or microbially derived N, rather than cover crop-derived N. Microorganisms in soil have been found to prefer microbial cell material over straw as a N substrate, even when the straw had a low C/N ratio of 14:1 (Jawson et al., 1989). Amato and Ladd (1980) found that isotopically labeled microbial biomass was a more labile N pool than plant residue. Dead microbial biomass has been identified as a primary component of native soil available N (Marumoto et al., 1982).
In another study on the Chualar soil, soil mixing and disruption of aggregates caused a sudden decline in MBN, as determined by fumigation–extraction, and in most microbial groups, as determined by phospholipid fatty acid analysis (Calderón et al., in press). In our study, soil disturbance during incorporation of cover crop residue into the soil may have killed a portion of the microbial biomass, thereby possibly producing a preferred N source to other microbes whose growth was stimulated by cover crop-derived C. This could partially account for the much lower 15N enrichment of the MBN compared with high enrichment of 15N in the NH+4–N and NO-3–N pools. This explanation is consistent with the idea that the microbial biomass that is newly formed from the decomposition of plant residue is partly associated with the residue and partly located within the soil matrix (Ladd et al., 1995), giving it close proximity to available N in the soil. Another source of soil-derived N may have been NO-3–N. Microbial immobilization of the large pool of unlabeled NO-3–N may have been stimulated by the temporary availability of cover crop-derived C (Jackson et al., 1989; Azam et al., 1993). In either of these scenarios, soil microbes would be relying on different substrates for C vs. N assimilation.
The microbes that were decomposing the fresh phacelia residue experienced conditions of relatively high moisture content. The soil was kept consistently moist. Phacelia leaves and stems are slightly succulent, and the added residue had a high water content. Shoot material contained 86% moisture. Water availability may have been important for the movement of organisms between the surfaces of soil particles and decomposing plant residue. Water films may have also promoted the transport and diffusion of water soluble carbohydrates and NO-3–N back and forth between phacelia residue and soil particles. The mixing of these compounds could have been facilitated by the sandy soil texture. Thus, one factor that might partially explain the microbial utilization of C and N substrates from different sources could be enhanced movement of organisms and decompositional products in water films.
Rapid mineralization of C and N is to be expected in sandy soils, where there is little clay and silt to protect microbial biomass and microbial decay products (van Veen et al., 1987; Ladd et al., 1995). Similar rates of net N mineralization of residue to this study were found in response to incorporation of 15N-labeled white mustard (Sinapis alba L.) catch-crop material (C/N of 15:1) in an agricultural field on a sandy loam soil (Jensen, 1992). More rapid rates (44% mineralized in only 18 d) were observed when crimson clover (Trifolium incarnatum L.) shoots were incorporated into a gravelly loam soil (Crozier et al., 1998). However, 15N-labeled leguminous material (C/N of 15:1) in a medic pasture mineralized N more slowly, and 15N in the MBN accounted for a higher proportion of added 15N than in our study (Ladd et al., 1981a). Their soil (Roseworthy) had fairly similar particle-size distribution (82, 4, and 12% sand, silt, and clay, respectively) and organic C (7.3 g kg-1) as the Chualar soil (Ladd et al., 1981a), but microbial biomass was approximately two times higher (Amato and Ladd, 1980). After 1 yr of decomposition, Ladd et al. (1981a) recovered 52% of the legume residue in the topsoil of the medic pasture, but 39% of the white mustard (Jensen, 1992) and 45% of the phacelia residue (this study) were recovered from agricultural soils. These are not large differences, but they tend to confirm that tilled soils, especially sandy soils, have less ability to retain N, possibly due to C limitation, compared with undisturbed agricultural or grassland soils where N retention is generally higher (Follett and Schimel, 1989; Biederbeck et al., 1994).
Nitrogen Fates and Losses
By the end of the first lettuce crop at 4 mo after cover crop incorporation, 40% of the 15N in the cover crop had been mineralized (Fig. 5). A substantial fraction was present in the lettuce crop, and a relatively low proportion was leached. The small percentage of the 15N that was not accounted for could have been lost by denitrification, or possibly may represent experimental error. Given the small amount of microbial immobilization of cover crop N and the propensity for leaching in this sandy soil, retention of 15N within the soil–plant system (0–60 cm) was quite high (>80%) during the first 4 mo after incorporation. Low rainfall in the spring contributed to the high recovery of 15N. In regions with higher spring rainfall, greater proportions of cover crop-derived N can be lost (Bremer and van Kessel, 1992; Harris et al., 1994). Although most of the cover crop-derived NO-3–N was retained within the rooting zone, nearly 5 g NO-3–N m-2 derived from soil and fertilizer were leached during the first crop of lettuce (Fig. 3). Most of this leaching occurred within a month of seeding, when lettuce plants had low N demand. Nearly 3 g NO-3–N m-2 were present in the soil column just before plant residues were incorporated; this would presumably have been lower if the cover crop had actually been grown in situ. In this study, fertilizer applications were split so that more N was available in the last wk prior to harvest in this slow-growing crop. Also, irrigation was applied to match crop demand (Gallardo et al., 1996). Appropriate scheduling of fertilizer and irrigation also undoubtedly contributed to the lower leaching rate later in the crop growth period.
Uptake of cover crop-derived N by the first lettuce crop was fairly high considering that it was harvested only 10 wk after planting. In several other studies using 15N-labeled leguminous residue with C/N ratios of 10:1 to 20:1, subsequent corn (Zea mays L.) and wheat (Triticum aestivum L.) crops recovered 8 to 28% of the added 15N (Ladd et al., 1981b; Varco et al., 1989; Harris and Hesterman, 1990; Crozier et al., 1998). These cereal crops were grown for periods of 4 to 7 mo, providing a longer period for plant uptake of mineralized 15N than occurs with lettuce. The second lettuce crop recovered a smaller fraction of cover crop-derived N than the first crop, and total 15N recovery in plants during the 7-mo period between cover crop incorporation and second crop harvest was 25% of the applied 15N.
The first crop of lettuce utilized 26 g N m-2, of which 23 g N m-2 was derived from non-cover crop sources. Fertilizer N can account for at most one-third of the lettuce N uptake. Mineralized N derived from sources of soil organic matter other than the cover crop must have been the major source of crop N. In fact, the importance of net mineralization and nitrification is demonstrated by the substantial NO-3–N produced during the subsequent 2 wk after cover crop incorporation. An increase of 8 g NO-3–N m-2 (0–60 cm depth) from non-cover crop sources occurred at this time, and may be partially attributed to N mineralization in response to soil disturbance and irrigation. Later studies have confirmed that soil mixing and aeration immediately stimulate NO-3–N production in this soil, even when plant residue is not simultaneously incorporated (Calderón et al., in press). Rewetting can also initiate a large burst of NO-3–N production (Davidson et al., 1993).
In summary, this study has shown that cover crop N became readily available in the sandy soil of this vegetable crop system. Although microbes mediate this process, little N from the plant residue was retained in the microbial biomass. Instead, much of the N that was released from the cover crop was rapidly nitrified, leading to NO-3–N accumulation in the soil. Irrigation and fertilizer management of the subsequent crop are consequently very important for maximizing plant uptake and minimizing the loss of NO-3–N derived from mineralization of N in the cover crop material.Calderón Jackson Scow Rolston 2000
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ACKNOWLEDGMENTS |
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Received for publication December 11, 1998.
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