Effects Of Plastic Mulch On Soil Nitrogen Cycling And Fate Of Fertilizer Nitrogen In Field-grown Maize At A Semiarid Site

A new cultivated pattern of two ridges and their furrows covered by plastic film has been widely applied in dryland farming systems, for its significant effect on increased yields and income in the semiarid Loess Plateau of china. Plastic mulch effect on the interaction between film fully-mulched ridge-furrow cropping and maize growth on temperature and moisture, soil nitrogen content, soil nitrogen cycling and the fate of fertilizer nitrogen in croplands has received limited attention. Four treatments were evaluated in2011and2012:(1) no mulch+no crop planting;(2) mulch+no crop planting;(3) no mulch+crop planting; and (4) mulch+crop planting. Narrow and wide ridges were alternated in all treatments and maize (Zea mays L.) was seeded in the furrows only for the crop-planted treatments. Main results as follows:1. Compared with no mulch, the presence of mulch in both the2011and2012cropping seasons consistently increased soil temperature by2.7-3.3℃in the0-20cm soil layer over the course of the whole experiment under no maize planting, whereas it increased by2.6-3.3℃in the first two months of the season only under maize planting. This showed that no increase in soil temperature in the later growth stages in mulched maize-planted fields was due to the interception of solar radiation by the crop canopy. In2011, mulch increased soil water storage (in the0-100cm depth) by19-40%and conserved100mm more rainfall in the0-170cm depth over the whole experimental course compared with unmulched treatment under no maize planting, However, soil water storage was higher in mulched treatment in the early two and half months but was similar in the remaining months under maize planting, and that a15%higher amount of water loss happened in mulched treatment than in unmulched treatment under maize planting. In2012, soil moisture dynamics under the four treatments were different from those in2011because of residue effects and higher precipitation. In2012, soil water storage was higher in mulched treatments than in unmulched treatments throughout the experimental course, regardless maize planted or not. Compared to no mulch, mulch increased grain yield and water use efficiency by70-72%and57-77%, respectively, over the two experiment years. The results suggest that, in addition to increased soil temperature during the early growth stages, increased transpiration under mulch contributes to increased grain yield and water use efficiency compared with no mulch.2. Two months after urea application, total NH3-N volatilization in unmulched fields (30-34kg ha-1) was ten times that in mulched fields. Over five months of maize growth, soil N in the0-15cm was mineralized in the first three months but immobilized in the later two months. N mineralization rate was generally unaffected in the first one month but stimulated by mulch in the middle two months; N immobilization rate was reduced by mulch in the last two months, compared with no mulch. Maize planting increased soil N mineralization mainly in the peak growth compared with no maize planting. Total N mineralized over season was201-309%greater in mulched than in unmulched soils and12-114%higher in maize-planted than in non-maize-planted soils, indicating that soil N mineralization was stimulated by the improved soil hydrothermal conditions under plastic mulch compared with no mulch. The reduced ammonia volatilization and stimulated N mineralization were responsible for the increased soil N content and uptake by maize under mulch compared with no mulch. Our results demonstrate that plastic mulch prominently changes N cycling in croplands.3.15N isotope traced results in field experiment showed that In2011, in-season maize use efficiency of fertilizer N was23%under no mulch and19%under mulch, that is, a19%decrease under the latter than under the former (P<0.001). In2012, maize use efficiency of residual labeled N (%of N from labeled N in aboveground biomass in2012over labeled-urea N applied in2011) was10%under no mulch and16%under mulch, an increase by63%under the latter than under the former (P<0.001). The cumulative maize N use efficiency (%of N from labeled urea in aboveground biomasses of2011and2012over labeled N applied in2011) was similar in non-mulched (34%) and mulched plots (36%)(P>0.05).At the time of harvest in2011, Total labeled N in soil under mulch was21and30%greater in non-maize-and maize-cropped plots, respectively, than under no mulch. Mineral labeled N amount in soil was greater under mulch than under no mulch whereas organic labeled N was similar between non-mulched and mulched treatments, regardless of whether maize was cropped. At the time of harvest in2012, the least amount of labeled N (50%of the applied) was retained in the170cm soil depth in the unmulched non-maize-cropped treatment. The content of labeled N presenting as mineral forms was significantly lower in mulched soils than in non-mulched soils whereas labeled N as organic forms was significantly greater in mulched than in non-mulched soils.Over the2011cropping season, the unaccounted labeled N was negligible under mulch (5%of the applied) whereas substantial amounts of labeled-N under no mulch (18-22%of the applied) was unaccounted in the plant-soil system, regardless of whether maize was cropped or not. Thus, it could be inferred that ammonia volatilization in season dominated over the loss of labeled N under no mulch, which, however, was eliminated under mulch. Over the time span between harvest in2011and in2012, without maize cropping unaccounted labeled-N was80%greater under mulch (46%of the applied in2011) than under no mulch (26%). With maize planting, the difference in the unaccounted labeled-N was similar in mulched and non-mulched soils. Maize cropping decreased unaccounted labeled-N compared with no maize cropping.In conclusion, the effect of plastic film mulch on soil temperature and soil water was depended on maize growing; The main reason of soil mineral nitrogen increasing was that plastic film directively inhibited ammonia volatilization and increased the rate of nitrogen mineralization; Fertilizer use efficiency was so low that a large amount of fertilizer nitrogen was residual in the soil, so it is necessary to adjust the amount of fertilizer application.