Abstract
The depletion of soil organic matter (SOM) reserve after deforestation and subsequent management practices are well documented, but the impacts of land-use change on the persistence and vulnerability of storage C and N remain uncertain. We investigated soil organic C (SOC) and N stocks in a landscape of chrono-sequence natural forest, grazing/crop lands and plantation forest in the highlands of North-West Ethiopia. We hypothesized that in addition to depleting total C and N pools, multiple conversions of natural forest significantly change the relative proportion of labile and recalcitrant C and N fractions in soils, and thus affect SOM quality. To examine this hypothesis, we estimated depletion of SOC and N stocks and labile (1 & 2) and recalcitrant (fraction 3) C and N pools in soil organic matter following the acid hydrolysis technique. Our studies showed the highest loss of C stock was in grazing land (58%) followed by cropland (50%) and eucalyptus plantation (47%), while on average ca. 57% N stock was depleted. Eucalyptus plantation exhibited potential for soil C recovery, although not for N, after 30 years. The fractionation of SOM revealed that depletions of labile 1 C stocks were similar in grazing and crop lands (36%), and loss of recalcitrant C was highest in grazing soil (56%). However, increases in relative concentrations of labile fraction 1 in grazing land and recalcitrant C and N in cropland suggest the quality of these pools might be influenced by management activities. Also, the C:N ratio of C fractions and recalcitrant indices (RIC and RIN) clearly demonstrated that land conversion from natural forest to managed systems changes the inherent quality of the fractions, which was obscured in whole soil analysis. These findings underscore the importance of considering the quality of SOM when evaluating disturbance impacts on SOC and N stocks.
Highlights
Deforestation is the dominant land-use change process, and results in severe impacts on the biogeochemical properties of soil
A landscape with four chrono-sequence land-use types viz. natural forest, grazing/crop lands and eucalyptus plantation was examined for storage and quality of soil organic C (SOC) and N as influenced by land conversions in Ethiopian highlands
Grazing land and cropland showed the highest loss of C and N stocks, respectively, compare to soils of original natural forest
Summary
Deforestation is the dominant land-use change process, and results in severe impacts on the biogeochemical properties of soil. The impacts occur directly through changes in above and below-ground vegetative inputs, turnover of soil organic matter and soil erosion; and indirectly, through biodiversity loss [1,2] All these processes have a regulatory influence on the storage of SOC and N, depending on various modifying factors such as climate, forest types, land and soil characteristics and post-conversion management activities. Most previous studies considered organic C stocks in bulk soil, despite the fact that soil organic matter (SOM) is composed of several pools, with different degrees of stabilization, and turnover times ranging between a few days to several centuries [5] It necessary to evaluate the quality or biodegradability of the remaining organic matter as influenced by land-use change
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