Assessment of changes in soil organic carbon fractions and enzyme activities under apple growing ecosystems in temperate North-Western Himalayas

Abstract

For better understanding about the dynamics of soil organic carbon (SOC) and enzyme activities in soil health management under intense cropping and fluctuating climatic scenario, we examined quantitatively the spatial distribution and relationship pattern of SOC (potassium dichromate oxidizable C (KCr2O7 -C), total organic carbon (TOC or SOC stocks) and SOC pools of labile organic carbon (LBC), particulate organic carbon (POC) and microbial biomass carbon (MBC) as well as enzyme activities viz:- dehydrogenase (DHA), urease (URE) and phosphatase (PPTase) under apple growing ecosystems in north western temperate Himalayas, India. Descriptive statistics, Pearson’s correlation and spatial variability analysis of SOC attributes and enzyme activities were performed. Analysis of the results show consistent decline in SOC with depth and altitude exhibiting distinct spatial variability (CV = 15.7%–44.4%); representing significantly greater proportion in orchards of higher altitude by a margin of 346% and 295% and surface layers by 32% and 68% than sub-surface layers of those mid and lower altitudes. SOC stocks ranged from 32.1 to 94.6 Mg ha−1 with significant variation (CV = 18.0%–30.9%) between soil depths. Amongst labile carbon fractions, LBC observed significantly maximum spatial variability (CV = 15.0%–27.2%) followed by MBC (CV = 7.7%–20.6%) while as POC represents least non-significant variation (CV = 7.2%–15.5%). Although LBC (0.78 to 4.90 g kg−1) accounts for significant higher fraction (490%), MBC represents substantial reduction of 36% to 71% from surface to sub-surface layers. Decrease in microbial quotient (MQ) from an average value of 4.0% (surface) to 2.7% (sub-surface) indicates distinct spatial variability (CV = 7.5%–17.8%). The stratification ratio (SR) of SOC changed significantly amongst three altitudes varying from 2.75 to 3.96. For soil enzyme activities tested, DHA, URE and PPTase observed significant decline with depth showing maximum activity loss of 504% for URE followed by 470% for PPTase and 310% for DHA. Relationship studies recorded significantly positive correlation (p =0.05 and 0.01) for TOC with SOC and POC (0.752* and 0.814**) and between SOC and POC (0.675*), although SOC was best correlated with LBC (0.834**) and MBC (0.650**). All carbon fractions showed strong positive and significant (p = 0.05 and 0.01) relation with MBC and all enzymes (DHA, URE and PPTase) exhibited similar relationship with SOC (r = 0.638*), (r = 0.725*) and (r = 0.502**). Principal component analysis (PCA) summarized the spatial variation and relationship among SOC attributes and enzyme activities. These results of soil quality signature can be used to design and implement effective management measures in terms of addition of appropriate nutrients and effective carbon sequestration practices for soil health sustainability.