Soil Organic Carbon, Hydro-Physical Properties, Pore Structure and Greenhouse Gas Fluxes under Integrated Crop-Livestock System

Integrated crop-livestock system (ICLS) can improve soil organic carbon (SOC), enhance soil hydro-physical properties, and mitigate greenhouse gas (GHG) emissions. The objectives of this study were to: (i) investigate the influence of grazing, cover crop (CC), and a combination of grazing and CC on labile soil carbon (C) and nitrogen (N) fractions and soil enzyme activity, (ii) assess the impacts of short-term (3-4 years) and long-term (\u3e30 years) ICLS on SOC, bulk density, structural quality and water infiltration, (iii) evaluate soil pore structural properties such as porosity, number of pores, pore size, pore shape, fractal dimension, anisotropy and tortuosity under contrasting management systems including ICLS using micro-computed tomography technique, (iv) evaluate the impacts of CC, short-term grazing of crop residues and CC on soil surface GHG emissions, and (v) compare temporal variations in measured daily nitrous oxide (N2O) fluxes, soil moisture and temperature under corn and soybean phases of a corn-soybeanoat rotation with predicted values from the Daycent model. Both short-term and longterm ICLSs consisted of a no-till corn-soybean rotation system using multi-species cover crops plus livestock grazing of cover crops and corn residue. These ICLSs were compared to the business-as-usual scenarios of conventional row-crop management (CNT, no-till corn-soybean rotation with winter fallow) and long-term grazed pasture (GP). Data showed that labile soil C and N fractions were higher under ICLS compared to the CNT soils only after long-term, but not under short-term management. Specifically, soils under long-term ICLS had higher hot water extractable organic carbon (HWC), cold water extractable organic carbon (CWC), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), potential carbon mineralization (PCM) and potassium permanganate oxidizable carbon (POXC) compared to the CNT. Long-term ICLS also enhanced β-glucosidase activity as compared to the CNT. Soils under GP always had higher C and N fractions than soils under ICLS and CNT. Soil organic C was positively correlated to all labile C and N fractions except POXC across all management systems. Long-term inclusion of cover crops and livestock in the traditional corn-soybean rotation improved soil physical and hydrological properties. Soil organic carbon was higher by 20-26% and bulk density was lower by 18-37% in long-term ICLS than that of CNT. Long-term ICLS also improved soil water retention, macroporosity, infiltration rate and total porosity as compared to the CNT. However, these properties were not affected by short-term ICLS. The GP always had the lower soil bulk density and higher SOC and infiltration rate as compared to the ICLS and the CNT. X-ray micro-computed tomography provides a detailed characterization of the soil porous system at the micrometric scale. The computed tomography-derived porosity was higher in native pasture (NP) (taken as a reference to compare effects of ICLS and CNT on soil pores) (12.8%) and ICLS (8.2%) compared to the CNT (4.3%). Fractal dimension was higher in NP (2.5) compared to ICLS (2.4) and CNT (2.3), indicating higher complexity of soil porous structure in NP and ICLS. Soils under CNT had larger values of degree of anisotropy and tortuosity as compared to the NP and ICLS, indicating the degradation of aggregate structure. Although total soil porosity was dominated by large pores (\u3e10 mm3) in all the management systems, the abundance of large pores was significantly higher in ICLS than the CNT. Triaxial shaped pores occupied a bigger fraction of porosity and number of pores for all the treatments which can enhance root penetration and transmission of water and gases through soils. Integrated crop-livestock system can be an effective strategy to mitigate soil surface GHG emissions. Grazed pasture increased cumulative soil carbon dioxide (CO2) fluxes by 34-57% and decreased nitrous oxide (N2O) fluxes by 26-50% as compared to the CNT management. Cumulative CO2 fluxes were similar among CNT, CC, and grazing (G) treatments. Cover crops inclusion after oats lowered cumulative N2O fluxes relative to CNT and G treatments. Cumulative methane (CH4) fluxes were not influenced by the treatments over the study period. Modeling results showed that Daycent accurately predicted soil temperature (r = 0.84 for corn and r = 0.77 for soybean), however, predictions for soil moisture content (r = 0.31 for corn and r = 0.22 for soybean) and N2O emissions (r = 0.37 for corn and r = 0.05 for soybean) were generally lower than the measured values during most of the experimental period. Overall, this study showed that long-term integration of CCs and livestock grazing of CCs and crop residue under ICLS can be beneficial in enhancing labile soil C and N fractions and soil physical and hydrological properties. Long-term adoption of ICLS improves soil pore properties which would enhance soil functional process (i.e. nutrient cycling, root growth, soil gas fluxes, water storage and availability.) Whether implementation of ICLS can improve other soil quality parameters and crop yield requires further investigation. One important short-term outcome was that the use of CCs and grazed pasture under ICLS decreased soil N2O emissions. Finally, the Daycent model needs further improvement to narrow the uncertainty range for accurate predictions of N2O emissions over a wide range of climate, soil, and management systems