Effect of fertilizer and organic matter inputs on nitrifier populations and N-mineralization rates in a dry tropical region, India

Abstract

The study was conducted in a dryland farm, which was under Oryza sativa (rainy season)/Lens esculenta (winter seasons) crop rotation, to assess the size of viable community of ammonia- and nitrite-oxidizing bacteria and N-mineralization, as affected by season and inputs of chemical fertilizer and organic matter. Six treatments with three replicates were established in completely randomized block design. The treatments were: control, chemical fertilizer, wheat straw (WS), WS + fertilizer, farmyard manure (FYM) and FYM + fertilizer. The effects of treatments and seasons on NH₄⁺--N, NO₃--N and PO₄--P pools were significant, but treatment × season interaction was not significant. The mineral N pools had a dry season maximum, while N-mineralization and nitrification rates were highest during rainy season and lowest during summer season. Differences in N-mineralization and nitrification rates due to treatments were significant. The N-mineralization rates were greatest in fertilizer and fertilizer + WS (or FYM) treatments and the nitrification rate was highest in the fertilizer treated soil. Analysis of pooled data of all treatments showed that N-mineralization rate was positively related with soil moisture and inversely with the size of mineral N pools. The numbers of viable cells of ammonia- and nitrite-oxidizing bacteria were related significantly to each other, and were highest during the rainy season and lowest in the summer season. The effect of treatments, except for the fertilizer-alone treatment, on the most probable number counts of the nitrifying bacteria was not statistically significant.