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

Abstract

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 resulting 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 × 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 fertilization. Regarding fertilizer treatments, cumulative N2O emissions did not differ between the straw treatments (∼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-fertilizer treatments where the highest emissions were found in the straw treatments (∼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.