Response of Contrasting Nutrient Management Regimes on Soil Aggregation, Aggregate-Associated Carbon and Macronutrients in a 43-Year Long-Term Experiment

The present investigation evaluated the effect of continuous application (>43 years) of organic and inorganic fertilisers on soil aggregate stability, aggregate size distribution, aggregate-associated carbon and its fractions, and total macro-nutrient content under the soybean-wheat cropping system in vertisols of the semi-arid region. Seven contrasting treatments consisted of T-1 (50% NPK), T-2 (100% NPK), T-3 (150% NPK), T-4 (100% NP), T-5 (100% N), T-6 (100% NPK + FYM) and T-7 Control (crop raised without addition of any nutrient). The highest and lowest percentage of large macroaggregates (11.3%) was found in T-6 and T-7 treatments. The NPK + FYM (T-6) treatments substantially increased the proportion of the macroaggregate fractions (>2 mm and 2-0.25 mm) than other treatments. However, different manure and fertilisation treatments did not affect the proportion of silt + clay aggregates. Long-term application of 100% NPK + FYM increased mean weight diameter (MWD) and stable water aggregates (WSA) by 35.7 and 6.01% over control. The aggregate-associated SOC followed the trend of large macroaggregates > microaggregates > small macroaggregates > silt + clay fractions. Application of long-term manure plus inorganic fertiliser (T-6) has also increased Walkley Black soil organic carbon (WBSC), permanganate oxidisable carbon (KMnO4-C), soil microbial biomass carbon (SMBC), carbon mineralisation (CM), total soil carbon (TSC), total soil N (TSN), total soil phosphorus (TSP) and total soil potassium (STK) by 82.1, 71.6, 182, 42.4, 23.9, 41.6, 117 and 18.4%, respectively, over control (T-7). The lowest metabolic quotient (MetQ) value of 5.13 mg CO2-C mg(-1) MBC h(-1) was obtained in the control treatment (T-7). The lowest MetQ was recorded in the integrated application of manure + inorganic fertiliser, i.e., 100% NPK + FYM (T-6). Similarly, microbial quotient (MiQ) was also higher in treatment T-6 (100% NPK + FYM) and lower in T-7 (control). It is concluded that the application of inorganic fertiliser alone is insufficient to maintain soil health and sustainability so, combined application of manure plus inorganic fertilisation is the most important nutrient management practice for long-term soil sustainability because it maintains SOC levels in soils for long periods and ultimately ensures the soil health of soybean-wheat cropping systems in the vertisols of semi-arid regions.