Effect of agricultural management on N2O emissions in the Brazilian sugarcane yield

The expansion of sugarcane cultivation in Brazil for sugar and bioethanol production has led to increased N-fertilizer use. Today, sugarcane is harvested mechanically and resultihg crop residues are retained as a mulch on the soil surface. We hypothesized that the combination of these activities (topdressing N fertilization applied on the mulching) promotes soil conditions that modify the microbiota involved in the soil N cycle, and consequently raise N2O emissions. We investigated a commercial sugarcane crop to determine whether a topdressing of N-fertilizer (100 kg N ha(-1) as ammonium nitrate) combined with sugarcane straw mulch (14 Mg ha(-1) dry mass) change soil attributes (pH, total C and N, microbial biomass C and N, inorganic-N and WFPS%), and the copy numbers of genes (nirS, nirK, norB and nosZ) involved in soil N-transformation with consequent increases in N2O emissions. The 3 x 2 factorial treatments were: three soil surface treatments: i) bare soil (no-straw); ii) sugarcane straw and, iii) synthetic straw (polypropylene strips) and with or without an application of N-fertilizer. The mulch treatments (sugarcane or synthetic) produced the highest emissions, which occurred at two `N2O hot moments' within 10 days after fertilizatioh. Regarding fertilizer treatments, cumulative N2O emissions did not differ between the straw treatments (similar to 99 mg m-2) but were higher than those of the no-straw treatments (51 mg m-2). Similar behavior was found in the no-fertili2er treatments where the highest emissions were found in the straw treatments (similar to 30 mg m-2) and loWest in the no-straW treatments (6 mg m-2). The copy numbers of the nirS, nirK, norB and nosZ genes were equal in the straw treatments, but were significantly lower in the no-straw. While high copy numbers of the norB gene were associated with the `N2O hot moments,' the same was not observed for the other genes. Redundancy analysis (RDA) indicated that N2O emissions were higher in relatiOn to microbial biomass and WFPS% than they were in relation to the norB gene and inorganic-N. Our findings show that N-fertilization combined with sugarcane-straw mulching raised N2O emissions by promoting a chain of interactions between soil attributes and microorganisms involved in N-transformation. (C) 2017 Elsevier Ltd. All rights reserved. (AU)