Ecosystem restoration of jhum fallows in northeast India: Microbial C and N along altitudinal and successional gradients

To characterize the altitudinal and successional trends in microbial biomass and to understand their role in soil nutrient dynamics during the aggradation phase (vegetation recovery) of abandoned shifting cultivation systems, we determined the soil properties and microbial C and N in jhum (slash-and-burn) cultivation systems at different altitudes and 1-, 7-, and 16-year-old fallow agricultural lands at lower and higher altitudes in the northeastern Indian hills. Density of ground vegetation was lower in the undisturbed forest than in the jhum fallows. In general, 1-year jhum fallow had greater herbaceous vegetation both at lower and higher altitudes. Although woody plants were observed in 7- and 16-year-old jhum fallows, their density was highest in the forest. Soil moisture, organic C, and total N also increased gradually with increasing altitude and progressive secondary succession. Soil pH showed a negative correlation with altitude (as also confounded by soil type) and fallow age. Both microbial C and N had a close correlation with altitude and fallow age. Contribution of microbial C to soil organic C was 2.0-2.6% and microbial N to total N 1.4-2.2% in jhum fields, 2.4-4.3% and 1.2-2.1%, respectively, in jhum fallows, and 2.5-2.9% and 1.6-1.9% in the forests. Microbial C and N showed a negative correlation with herbaceous plant density. Microbial biomass in the jhum fallows and forest stands had a positive relationship with woody vegetation. Along an altitudinal and/or successional gradient, microbial C and N were positively correlated with water-holding capacity, soil moisture, organic C, and total N and negatively correlated with soil pH. Microbial C and N were positively correlated with each other. Therefore, the study suggests that the altitudinal and successional dynamics of microbial C and N are linked to, among other properties, soil organic matter and total nitrogen contents in the soil during community development after land abandonment from shifting cultivation.