Black oat cover crop management effects on soil temperature and biological properties on a Mollisol in Texas, USA

Authors: Zibilske, Larry; Makus, Donald

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2009
Publication Date: 3/15/2009
Citation: Zibilske, L.M., Makus, D.J. 2009. Black oat cover crop management effects on soil temperature and biological properties on a Mollisol in Texas, USA. Geoderma. 149(3-4):379-385.

Interpretive Summary: In hot climates, there is a huge problem of keeping enough organic matter in the soil to sustain profitable agriculture. One way of adding more organic matter to soils is by growing cover crops when a cash crop is not grown, using a crop rotation. Much is not known about how best to manage the cover crop to ensure the most benefit to the soil and soil microbes so when the cash crop is grown again, it can be more productive and healthier. We set up a field experiment in which we grew a cover crop, black oats, and then tested three ways of dealing with the plants that grew to see which was better for the soil microbes. We did not mow some of the plants, or we mowed some all the way to the ground, and the rest we mowed about half way up the stems. During the summer, the temperature in the soil was higher under the mowed completely part, next highest under the partially mowed plants, and lowest in the sections not mowed at all. This is important because the lower the temperature, the lower the microbial activity and the more soil organic matter would be retained in the soil. We found that the enzymes that microbes use to decompose the crop supplied organic matter was highest in the not-mowed sections and lowest in the completely mowed sections. However, microbial mass was highest in the completely mowed sections. The important fertilizer element, nitrogen, became low in all the treatments due to plant material decomposition. Our results indicate that complete mowing is not the best way to deal with the plant material produced by the cover crop in a hot climate. Complete mowing keeps the soil too hot and the resulting high rate of organic matter decomposition depletes the soil of this vital resource. Overall, the not-mowed treatment was best in preserving soil organic matter, slowing down soil microbe growth and keeping the soil cooler.

Technical Abstract: This field experiment was conducted to evaluate effects of mowing (no mowing, flail mowing, or sickle mowing) management of a black oat (Avena strigosa [Schreb.]) cover crop on soil microenvironmental conditions and on microbial biomass, dissolved organic C (DOC), soil inorganic N, resin-extractable phosphate, alkaline phosphatase and ß-glucosidase activities during the hottest period of the year in a subtropical climate. Soil temperatures at depths of 0, 5, 10 and 20 cm were highest in flail mowed plots (near 45°C at 5 cm depth) , followed by sickle mowed plots (averaging 10°C lower at 5 cm depth). Lowest soil temperatures were seen in not-mown plots, averaging 5°C lower at 5 cm depth than in the sickle-mowed plots. Temperature converged below 5 cm, with the highest temperature (flail mowed) at 32°C and the lowest (uncut) at 27°C. Microbial biomass increased significantly (P<0.05) from <100 mg C kg-1 soil in the late spring, to >150 mg C kg-1 in the early fall. ß-glucosidase and alkaline phosphatase activities decreased slowly throughout the experiment, and were significantly greater (P<0.05) only when comparing the uncut treatment to both mowing treatments. Inorganic N decreased sharply, from approximately 0.025 mg N kg-1 soil in the uncut plots, to approximately 0.005 mg N kg-1 during a rainy period. Immobilization of N during rapid microbial biomass growth could also explain the decrease in inorganic N. While extractable P was significantly higher in the uncut treatment, few differences in extractable P were noted between either mowing treatment during the experiment. Results indicate that standing oat residues maintained higher levels of soil enzyme activities and the lowest microbial biomass. Limiting contact of crop residues with the soil may be a rate limiting factor for decomposition and suggests that residue decomposition may be delayed by practices that leave most of the plant biomass above the ground (i.e. uncut or sickle mowed). These factors may be important in managing soil nutrient transformations and soil organic matter maintenance in hot climates.