Impact of fertilization and grazing on soil N and enzyme activities in a karst pasture ecosystem

The biggest obstacle to economic development in karst areas comes from the ecological crisis, so healthy soil and adequate food are important. Soil nitrogen (N) is an important factor in maintaining soil quality. Understanding the changes in soil N fractions and enzyme activities under different management practices is important to maintain sustainable pastureland in karst areas. In this study, we investigated how grazing and fertilization affect different soil nitrogen fractions and enzyme activities of karst pastureland. The four treatments were unused land (UL, undisturbed utilization), abandoned-grazing pastureland (APL, grazing for three years followed by abandonment for six years), grazing pastureland (GPL, controlled grazing without fertilizer) and fertilized-grazing pastureland (FGPL, controlled grazing + fertilizer), with the aim of determining the management practices that is most beneficial to soil N in the area. Our results indicated that FGPL is the most beneficial in enhancing soil N fractions. This is reflected in greater total nitrogen (TN), alkaline driven nitrogen (NOH), inorganic nitrogen (TON), anaerobic mineralized nitrogen (The average increase in each soil layer over UL was 1.43 times; TMN), microbial biomass nitrogen (The average increase in each soil layer over UL was 3.51 times; MBN), amino acid nitrogen (AAN), amino sugar nitrogen (ASN), acidolyzed unknown nitrogen (TUN) and unacidified nitrogen (AIN) in FGPL soils. Likewise, greater enzyme activities were also found in nitrogen conversion-related enzymes (soil urease (S-UE), soil N-acetyl-β-D-aminoglucosidase (S-NAG), nitrate reductase (S-NR), and soil acid protease (S-ACPT)). In addition, the combination of grazing and balanced nutrient inputs slowed soil acidification and maintained high levels of total phosphorus (TP) and total potassium (TK). In contrast, the soil N fractions responded significantly to APL (low TN and plant effective nitrogen), while insufficient conversion capacity (enzyme activity, MBN) was the main reason for the compromised soil N availability at GPL. Government policy makers and livestock producers should encourage controlled grazing with proper fertilizer supplementation, and avoid prolonged abandonment of grazing lands.