Non-target impacts of pesticides on soil N transformations, abundances of nitrifying and denitrifying genes, and nitrous oxide emissions.

Agriculture is the leading contributor to global nitrous oxide (NO) emissions, mostly from soils. We examined the non-target impacts of four pesticides on N transformations, N cycling genes and NO emissions from sugarcane-cropped soil. The pesticides, including a herbicide glyphosate (GLY), an insecticide imidacloprid (IMI), a fungicide methoxy ethyl mercuric chloride (MEMC) and a fumigant methyl isothiocyanate (MITC), were added to the soil and incubated in laboratory at 25 °C. The soil microcosms were maintained at two water contents, 55 % and 90 % water holding capacity (WHC), to simulate aerobic and partly anaerobic conditions, respectively. Half of the soil samples received an initial application of KNO and were then maintained at 90 % WHC for 38 d, whilst the other half received (NH)SO and were maintained at 55 % WHC for 28 d followed by 10 d at 90 % WHC to favour denitrification. Responses of individual functional genes involved in nitrification and denitrification to the pesticides and their relationships to NO emissions varied with time and soil water. Overall, MITC had pronounced repressive effects on AOA and AOB amoA gene abundances and gross nitrification. Under 55 % WHC during the initial 28 d, NO emissions were low for all treatments (≤62 μg N kg soil). However, under 90 % WHC (either during the first 28 d or the increase in water content from 55 to 90 % WHC after 28 d) the cumulative NO emissions increased markedly. Overall, under 90 % WHC the cumulative NO emissions were 19 (control) to 79-fold (MITC) higher than under 55% WHC; with the highest emissions observed in the MITC treatment (3140 μg N kg soil). This was associated with increases in gross nitrate consumption rates and abundances of denitrifying genes (nirK, nirS and qnorB). Therefore, to minimise NO emissions, MITC should not be applied to field under wet conditions favouring denitrification.