Effects of Cover Measures on Soil Organic Nitrogen Fractions and Total Soluble Nitrogen Pools in Citrus Orchards of the Red Soil Hilly Region of Southern China

Soil organic nitrogen fractions and total soluble nitrogen (TSN) pools are crucial for assessing orchard soil fertility and tree nutrient requirements. Here, we studied the effects of grass cover (GC), plastic mulch (PM), and clean tillage (CK) on the physicochemical properties, organic nitrogen fractions, and TSN content of soil from a 0–60 cm depth in a 7-year-old orchard in the red soil hilly region of southern China. The results showed that GC and PM significantly increased the content of soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and total potassium (TK), as well as the C/N and N/P ratios. The mean total organic nitrogen (TON) content in the 0–60 cm soil profile was 1219.21 and 895.15 mg·kg−1 in the GC and PM treatments, and it was 67.36% and 22.88% higher in the GC and PM treatments than in the CK, respectively. In the 0–20 cm soil horizon, the active organic nitrogen (AN, AAN, ASN) content was 52.67% and 17.15% higher in the GC and PM treatments than in the CK, respectively. In the 20–60 cm soil horizon, the stable organic nitrogen (UN, NHN) content was 97.13% and 21.69% greater under the GC and PM treatments compared to the CK, respectively. Different ground cover methods increased the total soluble nitrogen (TSN) concentration in the 0–20 cm soil horizon while reducing it in the deeper 20–60 cm layer. Correlation analysis revealed significant positive correlations of SOC, TN, TP, TSN, and TON with acid-hydrolyzable nitrogen fractions (AN, AAN, ASN, UN) and significant negative correlations with bulk density (BD). Stepwise linear regression analysis, using the regression equation TSN = 0.372AN − 0.053NHN + 18.473 (p < 0.01, R2 = 0.925), identified AN as a critical indicator for TSN among the active organic nitrogen fractions.