Distribution of N-mineralizing Enzymes in Soil Aggregate Fractions over 46 Years Application of Inorganic and Organic Fertilizers in a Tropical Rice-Rice System

Abstract

The stability of enzymes in soil has been attributed to enzyme association with organic matter and the protection provided within soil aggregates. Enzymes namely urease, amidase and protease significantly affects nitrogen (N) mineralization and their assessment is crucial to study the nutrient cycling. Therefore, the objective of this study was to evaluate the hypothesis that the long-term application of farmyard manure (FYM) and inorganic fertilizers (N-nitrogen; P-phosphorus; K-potassium) impact the distribution pattern of enzymes namely, urease, amidase and protease in different fractions of water stable soil aggregates, and whole soil at 0–15 cm and 15–30 cm soil depth. The treatments comprised of unfertilized control and different combinations of inorganic fertilizers and FYM viz. control, N, NP, NK, NPK, FYM, FYM+N, FYM+NP, FYM+NK and FYM+NPK. A significant difference in soil aggregate size distribution was observed at two sampling depths. Total water stable aggregates (WSA) ranged between 69.8–91.2% in which 0.1–0.053 mm aggregate fraction contributed (2.11–3.87%), whereas 0.25–0.5 mm aggregate fraction was having the highest (27.3–32.6%) contribution. The activities of three enzymes in whole soil as well in aggregate fractions were lowest in control and highest in FYM+NPK except for amidase, which was having highest activity in FYM alone treatment. Activities of all the three enzymes were highest in aggregate fraction of 5–2 mm. Activities of three enzymes in whole soil as well as in aggregate fractions were lower at 15–30 cm compared to 0–15 cm soil depth. It may be concluded from this study that long-term addition of FYM alone or in combination with inorganic fertilizer increases the macroaggregate (5–2 mm) and hence the overall activities of N mineralization enzymes.