Soil redistribution impacts on the spatial variation of nutrients, net carbon exchange with the atmosphere and soil respiration rates in highly eroding agricultural fields from the foothills of the Indian Himalaya.
Date: 17 June 2016
University of Exeter
PhD in Geography
Abstract Using the tracer caesium-137 (137Cs) and experimental approaches this study quantified soil redistribution induced spatial variation of nutrients and soil organic carbon (SOC), net C flux between soil and atmosphere and soil respiration rate at various landscapes positions (eroding to deposition) within agricultural fields ...
Abstract Using the tracer caesium-137 (137Cs) and experimental approaches this study quantified soil redistribution induced spatial variation of nutrients and soil organic carbon (SOC), net C flux between soil and atmosphere and soil respiration rate at various landscapes positions (eroding to deposition) within agricultural fields from the foot hills of Indian Himalaya. The depth distributions of 137Cs and the spatial patterns of 137Cs inventories were consistent with previous applications of the approach in that low inventories were associated with low concentrations in the cultivation layer and high inventories were reflected in deeper 137Cs profiles indicative of accumulation of labelled soil. This supports the contention that 137Cs is a suitable tracer for use in this environment. The study found that soil redistribution within fields altered the spatial variation of nutrients and SOC; with significantly lower concentrations of nutrients in the most eroded part of fields (upslope) and significantly higher concentrations of nutrients and SOC in the depositional part of field (downslope). The spatial pattern of nutrients and SOC is reflected in differences in depth distributions between eroded and depositional areas. The 137Cs and SOC inventory and depth distribution data were used to derive retrospective assessments of net C exchange between soil and atmosphere. The C flux quantification model was used to estimate lateral and vertical soil and SOC redistribution under an assumption of equilibrium conditions and the net exchange of C between soil and atmosphere was derived from the difference between measured and ‘equilibrium’ SOC inventories. Fluxes were derived for each landscape position within the agricultural fields studies and calculated at field and site scale. High rates of soil loss were measured and the results showed that the majority of eroded sediment and SOC was exported from field with only a small fraction redeposited within the field. The effect of soil and SOC redistribution was to create disequilibrium in SOC dynamics at eroding and deposition positions and this supported the formation of a field scale C sink. The sink strength is highest in the most eroded parts of the fields due to dynamic replacement of eroded C. This is assumed to be due to the high rate of incorporation of SOC-poor subsoil, with a large C-unsaturated surface area, into the cultivation layer. The C sink is smaller that those reported from high nutrient-input mechanised farm lands. Irrespective of the fate of exported SOC, the SOC stocks in the fields appear to be in dynamic equilibrium and, therefore, there is no evidence of a C source to the atmosphere due to erosion. Also the rate of SOC export from the fields is very high, especially when compared with mechanised fields and, if it is assumed that some portion of exported C is stored in some part of low lying area, the C sink strength would be comparable to mechanised farm lands. The soil redistribution and C flux study confirmed the existence of spatial variation in C flux at various landscapes position and was consistent with an important role for vertical mixing of soil and SOC in determining net C exchange with the atmosphere. This informed the design of the final element of the research that examined soil respiration differences in soil from shallow and deep layers in eroding and aggrading landscapes position. Respiration was measured over a one year period in samples derived from separate depth layers and in mixtures of soil from different depths at each landscape position. No significant difference was found in C release rate (per unit mass of C) from topsoil of eroding and deposition position but the subsoil of eroding pits exhibited significantly higher C release than the subsoil from deposition positions. This result suggests that topsoil in both locations has almost equal and similar C origin. The relatively high rate of respiration in sub soils from eroding pits may be due to the presence of a larger proportion of SOC formed from recently incorporated plant material (crop roots) at these locations. In buried and deposition locations the reduced mineralisation is consistent with the proposition that burial of top soil can contribute to formation of a C sink. In the samples containing mixed topsoil and subsoil, evidence for priming was seen where the respiration rate in the mixed sample was significantly higher than the expected rate based on the respiration rate seen in the separate depth samples. No priming was evident in mixed soils from eroding locations, suggesting that mixing of subsoil and surface soil does not accelerate loss of old SOC from the subsoil. In contrast, significant priming action was evident in mixed soils from aggrading locations suggesting that buried SOC at depositional locations may be subject to accelerated respiration as long as it is exposed to fresh plant input (as found in surface soils). In conclusion, despite the low input and low productivity of the farmlands in the Indian Himalaya region studied here, there is consistent evidence that high rates of soil erosion and soil redistribution have induced spatial variation of nutrients and SOC, net C flux and soil respiration rates that combine to create a pattern of SOC stocks that are close to equilibrium and, if some of the exported C is sequestered, to create a net C sink. This result again confirms that erosion induced redistribution of C does not directly cause a net release of C to the atmosphere. The consistency of these results with previous studies suggests that there is both scope and need for soil erosion induced carbon fluxes to be incorporated into carbon budgets, research frameworks, land management and climate change mitigation strategies at policy-relevant scales.
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