IntroductionNitrous oxide (N2O) emission from soil is a major concern due to its contribution to global climate change and its function as a loss mechanism of plant-available nitrogen (N) from the soil. This is especially true in intensive agricultural soils with high rates of N fertilizer application such as those on the semi-arid Southern High Plains, USA.MethodsThis study examined emissions of N2O, pore-space concentrations of N2O and nitric oxide (NO), soil chemical properties, water content, and the genetic potential for N cycling five years after conservation system and N management implementation.ResultsFor these semi-arid soils with low N, carbon, and water contents, large soil N2O emissions (up to 8 mL N2O-N m-2 day-1) are directly related to the application of N fertilizer which overwhelms the N2O reducing capacity of the soil. When this fertilizer N is depleted, N2O flux is either low, non-existent, or net-negative and has been observed as early as mid-season for preplant applied N fertilizer (-0.1 mL N2O-N m-2 day-1). Soil pore-space gas concentrations (N2O and NO) remained relatively constant across the growing season (average N2O: 0.78 µL N2O L-1 soil air; NO: 3.3 µL NO L-1 soil air, indicating a base-level of N-cycle activity, but was not directly related to surface emissions of N2O which decreased across the growing season. In addition, genetic potential for N cycle activities increased across the growing season simultaneously with stagnant/reduced N cycle activity. This reflects the difficulty in relating genetic potential to in-situ activity in field research.ConclusionIt is likely that in a nutrient and carbon-poor soil, such as the semi-arid agricultural soil in this study, the microbial processes associated with N cycling are mostly limited by inorganic-N and less directly related to genetic potential at the time of sampling.
The Rolling Plains of Texas are historically semi-arid with sporadic, high intensity storms followed by periods of long drought. Fallow management is a common practice intended to store soil water, but leaves the bare soil exposed to erosive forces that can diminish the soil productivity. Cover crops in no-till (NT) agriculture have been proposed to increase soil health under environments with low precipitation as an alternative to fallow management. This study evaluated multiple treatments in a dryland cotton system including: (1) conventional tillage (CT); (2) NT; and NT with the following cover crops: (3) wheat; (4) Austrian winter pea; (5) crimson clover; (6) hairy vetch; and (7) mixed species cover. Soil samples were collected at 0, 3, and 6 weeks after cover crop termination and analyzed for soil organic carbon, total nitrogen, inorganic N, water-extractable organic carbon, water-extractable organic nitrogen, carbon mineralization, and phospholipid fatty acid analysis. For all parameters tested, there was no significant difference between CT and NT at any date or depth, so the addition of cover crops to NT cotton systems might be needed in order to enhance NT in regard to soil function. The multi-species mixed treatment was predicted to perform the best out of the cover crop treatments due to its combined benefits from grasses and legumes. However, the single-species Austrian winter pea treatment had 24% and 28% higher soil carbon and nitrogen than no-till without a cover crop, and can be a useful alternative to fallow management under these dryland agriculture conditions.
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