RESUMO
Estimating lateral carbon fluxes in agroecosystems presents challenges due to intricate anthropogenic and biophysical interactions. We used a modeling technique to enhance our comprehension of the determinants influencing lateral carbon fluxes and their significance in agroecosystem carbon budgets. The SWAT-C model was refined by incorporating a dynamic dissolved inorganic carbon (DIC) module, enhancing our ability to accurately quantify total lateral carbon fluxes. This improved model was calibrated using observed data on riverine particulate organic carbon (POC) and dissolved organic carbon (DOC) fluxes, as well as net ecosystem exchange (NEE) data monitored by a flux tower situated in a representative agricultural watershed, the Tuckahoe Watershed (TW) of the Chesapeake Bay's coastal plain. We assessed the losses of POC, DOC, and DIC across five primary rotation types: C (continuous carbon), CS (corn-soybean), CSS (corn-soybean-soybean), CWS (corn-wheat-soybean), and CWSCS (corn-wheat-soybean-corn-soybean). Our study revealed notable variations in the average annual fluxes of POC (ranging between 152 and 198 kg ha-1), DOC (74-85 kg ha-1), and DIC (93-156 kg ha-1) across the five rotation types. The primary influencing factor for annual POC fluxes was identified as sediment yield. While both annual DOC and DIC fluxes displayed a marked correlation with surface runoff across all crop rotation schemes, soil respiration also significantly influenced annual DIC fluxes. Total lateral carbon fluxes (POC + DOC+DIC) constituted roughly 11 % of both net ecosystem production (NEP) and NEE, yet they represented a striking 95 % of net biome production (NBP) in the TW's agroecosystem. Grain yield carbon accounted for 80 % of both NEP and NEE and was nearly seven times that of NBP. Our findings suggest that introducing soybeans into cornfields tends to reduce NEP, NEE, and also NBP. Conversely, integrating winter wheat into the corn-soybean rotation significantly boosted NEP, NEE, and NBP values, with NBP even surpassing the levels in continuous corn cultivation.
RESUMO
Excessive quantity of nutrient produced from various point and non-point source deteriorates the quality of water. For reduction of nutrient from a watershed, it is needed to be quantified followed by implementation of appropriate management practices. In this study, major sources of nutrient in Big Sunflower River Watershed (BSRW) were identified, quantified, and Soil and Water Assessment Tool (SWAT) model was applied for assessment of flow, sediment and nutrient. SWAT was calibrated and validated for streamflow, sediment, total nitrogen (TN), and total phosphorus (TP) for three United States Geological Survey (USGS) gauge stations within BSRW. Moreover, different scenarios, based on agricultural operation and best management practices, were developed. Performance of SWAT was evaluated using Nash-Sutcliffe Efficiency (NSE) and coefficient of determination (R2). The performance was good for streamflow during calibration and validation with R2 and NSE ranging from 0.74 to 0.90 and 0.70 to 0.82 respectively. SWAT performed satisfactorily for sediment, TN and TP except in few extreme conditions, where animal waste was mixed with farm runoff. This study has provided a suitable crop rotation and management practice for efficient management of nutrient in BSRW. Manure applied on field cultivated entirely with soybean was the best practice for reduction of TN with R2 and NSE ranging from 0.70 to 0.90 and 0.58 to 0.76 respectively. Application of manure only on existing soybean crop-land was the best practice for reduction of TP with R2 and NSE ranging from 0.51 to 0.69 and 0.41 to 0.60 respectively. Soybean was effective in accumulating both nitrogen and phosphorus from soil. This study will be helpful for efficient planning and management of nutrient through suitable crop rotation and management practice.
