Effects of Long-Term Multi-Treatment Experiments on Organic Matter and Enzymatic Activity in Sandy Soil

This study shows an evaluation of the condition of organic matter against enzymatic activity in soil. Long-term static field experiments with fertilisation with manure (FYM), different minerals, and mineral–manure were used for the research. Assays were obtained of the content of total organic carbon (TOC), dissolved fraction (DOC), susceptibility to oxidation (CL1) and (CL), total nitrogen (TN), dissolved nitrogen fraction (DTNT), and available forms of potassium, phosphorus, and magnesium. The activity of enzymes dehydrogenases, catalase, β-glucosidase, proteases, alkaline, and acid phosphatase was determined. We calculated the enzymatic indices and those evaluating the labile organic carbon management (CMI and CPI) in soil. An increase in TOC, up to 8.85 g kg−1 and 8.56 g kg−1 (FYM, FYM + KN), respectively, as compared with the control (5.67 g kg−1), did not have a significant effect on the content of labile carbon fraction CL for the fertilisation treatments. Only a higher CL content was found in the soil with the FYM + PN and FYM + NPK + Mg treatments (2.07 g kg−1 and 2.05 g kg−1). All the fertilisation treatments under study demonstrated a decrease in the value of the carbon management index (CMI). Similar DOC values (on average, 75.14 mg kg−1) were noted. The average percentage share of the DOC fraction accounted for 1.163% TOC, and it was lower as compared with the control variant (1.33% TOC). The mineral fertilisation treatments decreased soil enzyme activities. Multiparametric enzymatic soil fertility indices differed due to soil properties, depending on the fertilisation applied.