Cover Crop Impacts on Soil Microbial Populations and Nitrogen Cycling of Dryland Agricultural Production in the Texas Rolling Plains
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The Rolling Plains of Texas are characteristic of a low input, high risk environment with semi-arid conditions and sporadic, high intensity storms followed by periods of long drought. Agricultural practices that increase input costs are not readily adopted in the region; however, better management practices might be needed to tolerate global climate change. Cover crops in no-till agriculture have been used to increase soil health under environments with low precipitation and serve as an alternative to fallow management. The first study took place in a dryland cotton system with evaluated treatments that include 1) conventional tillage (CT); 2) notill (NT); and no-till with the following cover crops 3) wheat; 4) Austrian winter pea (AP); 5) crimson clover; 6) hairy vetch (HV); and 7) mixed species (MC). It was predicted that cover crops would improve soil health by increasing carbon mineralization (CMIN) and increasing soil carbon (C) and nitrogen (N) pools. Soil organic carbon (SOC) and total N (TN) were highest in AP and MC treatments. The water extraction technique, which measures the smaller, labile subset of the nutrient pool, proved to be a more sensitive measure of soil substrate. Total NO3 - , water-extractable organic nitrogen (WEON), water-extractable organic carbon (WEOC), and CMIN levels were significantly higher in no-till treatments with cover crops (AP or HV) when compared to no-till without cover crops. The total phospholipid fatty acid (PLFA) biomass analysis shared this same trend, but there were no significant treatment differences. Ammoniaoxidizing bacteria, responsible for nitrification, was highest in AP, and was highly correlated to total NO3 - . The mixed-species cover crop improved soil health more than CT and NT, but this study concluded that the single-species legume crop, AP, had the highest overall soil health improving benefits. Both AP and MC could be used as an alternative to fallow management. My second study took the alternatives to fallow a step further and tested a dryland continuous winter wheat with fallow management with two different cover crop management strategies. One subset of mix-species cover crops treatments were chemically terminated and left in the field to serve as ground cover and decomposable organic matter. Those treatments included in the mix-species treatments were divided in two more categories depending on seeding rates, which were 16.8 kg/ha (Low Mix) and 22.4 kg/ha (High Mix). The Low Mix and High Mix were further divided by termination timing, which were early maturity (55-70 days) and late maturity (75-90 days). A separate subset of treatments, cowpea, mung bean, and guar, were harvested to serve as a double crop for additional production income. The final treatments were a wheat-fallow and a wheat-canola (no cover crop) management as a control. Different offseason cover crop treatments were analyzed for CMIN, PLFA biomass abundance, water-extractable organic nutrients, inorganic N rates, and soil water storage to determine if the cover crop would be viable as a harvested double crop. Harvested guar reported the highest WEON, NO3 - and inorganic N values in the soil compared to the wheat-fallow management, while having statistically the same amount of stored water at the time of winter wheat being planting. The CMIN and WEOC did not statistically increase or decrease for any treatment when compared to fallow, which implies that CMIN and WEOC were not negatively impacted from changing management practices to fallow alternatives. Throughout all the experiments implemented, CT and NT never once indicated a significant difference between them at any date or depth, so the addition of cover crops to no-till cotton systems could potentially enhance no-till in regard to soil function. This research suggests that double crops could be a competitive strategy in the Southern Great Plains dryland wheat system.
water-extractable organic carbon
water-extractable organic nitrogen
Hux, Brian Adam (2019). Cover Crop Impacts on Soil Microbial Populations and Nitrogen Cycling of Dryland Agricultural Production in the Texas Rolling Plains. Master's thesis, Texas A&M University. Available electronically from