Seasonal Dynamics of Fertilizer-Derived Hydrolysable NH3 in an Arable Soil of Northeast China

Fertilizer applications to soil are widely known to be the most important anthropogenic sources to influence soil N turnover in agricultural ecosystems. More information is required on the relationships between soil organic N (SON) forms in order to predict the maintenance, transformation and stability of soil N. Accordingly, 15N-labeled (NH4)2SO4 (totally 200 kg N/ha) was applied to a maize crop throughout the entire growing period to investigate the distribution and the dynamics of fertilizer-derived N in hydrolyzable-NH3 fraction by measuring the labeled N in them. The accumulation of 15N in hydrolyzable-NH3 fraction was time-dependent although the total N concentration changed only slightly. The transformation of the residual fertilizer N to hydrolyzable-NH3-15N was maximal during the silking and grain filling stages, suggesting the fertilizer N was immobilized at an early stage during the growing period. The rapid decrease of 15N in hydrolyzable-NH3 pool indicated that hydrolyzable-NH3-15N was a temporary pool for fertilizer N retention and was able to release fertilizer N for uptake by the current crop

Nitrate leaching from residual fertilizer N after spring thaw in two corn agro-ecosystems

Water samples at zero tension were collected using an open-ended lysimeter and analyzed for NO3−-N from a Chicot sandy clay loam and a Ste. Rosalie clay soil under continuous corn (Zea mays L.) in 1993 and 1994, shortly after spring thaw. There was negligible leaching of NO3−-N at previous fertilizer N rates of 0 and 170 kg ha-1 in both soils. However, NO3−-N concentrations of the leachates from soils receiving 285 and 400 kg N ha-1 yr-1 varied from 1.4 to 80 mg L-1, depending on the initial levels of soil residual NO3−-N and the supply of percolation water. When the initial levels of soil NO3−-N were relatively high and percolation of water was relatively slow in 1993, NO3−-N concentrations of the leachates ranged from 20 to 80 mg L-1. Nitrite-N concentrations were from 1.4 to 15.6 mg L-1 when the initial levels of soil residual NO3−-N were relatively low and percolation was relatively fast in 1994. The occasional higher NO3−-N concentrations in the leachate from the previous higher N applications indicated a potential for contaminating surface and ground waters as a result of NO3−-N leaching in the early spring. Key words: Residual N, nitrate-N leaching, soil solution, continuous corn, N fertilization

Uptake and residual value of 15N-labelled fertilizer applied to first and second year grass seed crops in New Zealand

This study was established to quantify the uptake of 15N-labelled nitrogen (urea) applied in the first and second years of perennial ryegrass (Lolium perenne L.), tall fescue (Festuca arundinacea Schreb.) and browntop (Agrostis capillaris L.) seed crops, and the availability of the residual fertilizer N to a subsequent wheat (Triticum aestivum L.) crop under field conditions in Canterbury, New Zealand. Total recovery of 15N-labelled nitrogen fertilizer was approximately 100% when fertilizer was applied to the grass seed crops in spring. At harvest in year 1, grass straw and seed contained 34–47% and 6–15% of the applied N respectively; 27–35% remained in the soil (0–150 mm depth). Recovery of 15N in straw and soil was higher in fescue and ryegrass than in browntop, but recovery in roots was lower. At harvest in year 2, most of the 15N was present in the soil (30–37%) with only small amounts in the seed (0·7–1·0%), straw (3·6–4·9%) and roots (5·2–12·7%). In year 3, 2·5–3·5% of the residual 15N was recovered in the wheat and 18–26% in soil. Losses of 15N were minimal until ploughing after the second harvest, when there was an apparent loss of 11–35% of the fertilizer N applied. Losses were not directly associated with the fertilizer but indirectly following release of fertilizer N previously immobilized in plant roots and soil microorganisms. Small losses also occurred directly from autumn-applied N, probably through leaching. Despite these losses, overall there was an accumulation of fertilizer N in the soil organic pool, suggesting that ryegrass fescue and browntop seed crops have a role in contributing to the N fertility of the soil.

Effects of 15N-labelled crop residues and management practices on subsequent winter wheat yields, nitrogen benefits and recovery under field conditions

Nitrogen-15 enriched ammonium sulphate was applied to micro-plots in a field in which two leguminous (white clover and peas) and two non-leguminous (ryegrass and winter wheat) crops were grown to produce 15N-labelled crop residues and roots during 1993/94. Nitrogen benefits and recovery of crop residue-N, root-N and residual fertilizer-N by three succeeding winter wheat crops were studied. Each crop residue was subjected to four different residue management treatments (ploughed, rotary hoed, mulched or burned) before the first sequential wheat crop (1994/95) was sown, followed by the second (1995/96) and third wheat crops (1996/97), in each of which residues of the previous wheat crop were removed and all plots were ploughed uniformly before sowing. Grain yields of the first sequential wheat crop followed the order: white clover > peas > ryegrass > wheat. The mulched treatment produced significantly lower grain yield than those of other treatments. In the first sequential wheat crop, leguminous and non-leguminous residues supplied between 29–57% and 6–10% of wheat N accumulated respectively and these decreased with successive sequential crops. Rotary hoed treatment reduced N benefits of white clover residue-N while no significant differences in N benefits occurred between residue management treatments in non-leguminous residues. On average, the first wheat crop recovered between 29–37% of leguminous and 11–13% of non-leguminous crop residues-N. Corresponding values for root plus residual fertilizer-N were between 5–19% and 2–3%, respectively. Management treatments produced similar effects to those of N benefits. On average, between 5 to 8% of crop residue-N plus root and residual fertilizer-N was recovered by each of the second and third sequential wheat crops from leguminous residues compared to 2 to 4% from non-leguminous residues. The N recoveries tended to be higher under mulched treatments especially under leguminous than non-leguminous residues for the second sequential wheat crop but were variable for the third sequential wheat crop. Relatively higher proportions of leguminous residue-N were unaccounted in ploughed and rotary hoed treatments compared with those of mulched and burned treatments. In non-leguminous residue-N, higher unaccounted residue-N occurred under burned (33–44%) compared with other treatments (20–27%).

Winter Cover Crops and Nitrogen Management in Sweet Corn and Broccoli Rotations

Cover crops hold potential to improve soil quality, to recover residual fertilizer N in the soil after a summer crop that otherwise might leach to the groundwater, and to be a source of N for subsequently planted vegetable crops. The objective of this 5-year study was to determine the N uptake by winter cover crops and its effect on summer vegetable productivity. Winter cover crops [red clover (Trifolium pratense L.), cereal rye (Secale cereale L. var. Wheeler), a cereal rye/Austrian winter pea (Pisum sativum L.) mix, or a winter fallow control] were in a rotation with alternate years of sweet corn (Zea mays L. cv. Jubilee) and broccoli (Brassica oleracea L. Botrytis Group cv. Gem). The subplots were N rate (zero, intermediate, and as recommended for vegetable crop). Summer relay plantings of red clover or cereal rye were also used to gain early establishment of the cover crop. Cereal rye cover crops recovered residual fertilizer N at an average of 40 kg·ha-1 following the recommended N rates, but after 5 years of cropping, there was no evidence that the N conserved by the cereal rye cover crop would permit a reduction in inorganic N inputs to maintain yields. Intermediate rates of N applied to summer crops in combination with winter cover crops containing legumes produced vegetable yields similar to those with recommended rates of N in combination with winter fallow or cereal rye cover crops. There was a consistent trend (P < 0.12) for cereal rye cover crops to cause a small decrease in broccoli yields as compared to winter fallow.

DOUBLE-CROP FALL CABBAGE AS A SCAVENGER OF RESIDUAL FERTILIZER NITROGEN

High-value crops (tobacco and sweet corn) often receive high levels of N fertilizer during the growing season rather than risk yield and/or quality reductions. Following harvest, small-grain winter cover crops are sown to reduce soil erosion and recover residual fertilizer N. Fall cole crops, such as cabbage, grow rapidly in early fall, respond well to N fertilization, and have the potential to be sold for supplemental income. The objectives of this study were to 1) compare fall cabbage and winter rye as scavengers of residual fertilizer N and 2) determine if a relationship between fall soil mineral-N (NO–3 +) levels and fall cabbage yield response to N fertilization exists. Soil mineral N levels following sweet corn and tobacco ranged from 22 to 53 mg·kg–1 in the surface 30-cm and declined with depth. Fall cabbage appeared to be as effective as rye at reducing soil mineral N levels. No fall cabbage dry matter yield response to applied N was measured in 1993 and 1995. However, following sweet corn in 1994, a small cabbage yield response to N at 56 kg·ha–1 was measured when the soil mineral level, prior to fall fertilization, was 22 mg·kg–1.

OVERWINTER LOSSES OF NITROGEN-15 LABELLED UREA FERTILIZER

Field studies using 15N microplots were conducted to quantify the uptake and disappearance of fall- and spring-applied urea N on low organic matter, irrigated soils. Urea was mixed with the surface soil to maximize the potential for overwinter nitrification and subsequent losses of the fertilizer N. In three irrigated soils, losses of fall-applied urea averaged 24–31% compared with 11–21% of that applied at seeding. Barley took up 33–42% of spring-applied urea N but only 16–36% of fall-applied urea N. The lower uptake of fall-applied N apparently resulted from higher N losses rather than from the immobilization of fall-applied urea. Fall application resulted in lower soil reserves of residual fertilizer N after the growing season, as compared to spring application, in two of the three studies. Sixty percent of the fertilizer N recovered from the soil remained in the surface 15 cm. The application of 50 mm of water in the fall or 100 mm in the early spring, to intensify any effects of moisture, had a minimal effect on N losses or the distribution of N in the soil. This suggests that an individual rainfall event would not greatly affect the uptake or losses of fall-applied fertilizer on well-drained soil. The observed fertilizer losses, however, support practices such as concentrating fall-applied fertilizers in bands or the use of nitrification inhibitors. Key words: Denitrification, nitrogen, fertilizer, N balance, N losses, urea