Regulation of potential denitrification by soil on in long-term fertilizedarable soils

The denitrifying enzyme activity (DEA), denitrification potential (DP) and anaerobic respiration (RESP) together with chemical characteristics were me asured in three contrasting soils collected from experimental arable plots that had been subjected to long-term (21-23 years) fertilizer treatments. T he plots sampled were either unfertilized or had received either annual ino rganic NPK, manure and lime, or inorganic NPK and manure treatments. Additi on of inorganic NPK, manure and lime led to large increases in the DEA for two of the three soils, but in the absence of lime, inorganic NPK and manur e caused only small increases in DEA compared to unfertilized soils. Both D P and RESP were increased by the addition of inorganic NPK, manure and lime , but were substantially decreased by fertilizer treatments without lime. I n most cases there was a simple relationship between soil pH and either DEA and DP, with those treatments that reduced soil pH also leading to reduced denitrification and vice versa. The effects of artificially increasing the pH to a value close to the pH in unfertilized soils (6.3) by addition of N aOH to the soils that had received inorganic NPK, and which had the lowest soil pH values, were to increase substantially DEA, DP and RESP. In soil fr om one of the sites that had been stored for 5 weeks, the DP values respond ed differently between the fertilizer treatments. The DP value was lowest i n the soil that had inorganic NPK and manure, higher in the soil that recei ved inorganic NPK, manure and lime and it was the highest in unfertilized ( control) soil. The soil pH values for these treatments were 4.47, 5.79 and 6.58, respectively. However, when the soil pHs were adjusted by addition of either H2SO4 or NaOH to give a range between pH 2 and 12, the DP values fr om all three fertilizer treatments showed almost identical responses. The o ptimum pH value for DP was between 7 and 8 for all three fertilizer treatme nts. Substrate-induced respiration values from all fertilizer treatments sh owed a similar trend to DP when the soil pHs were modified. The results sho w that soil pH was an important factor which in the studied soils controls the microbial community in general and the community of denitrifiers in par ticular. However, denitrifiers showed a high pH resilience leading to no ma rked change of the pH optimum for potential denitrification.