Nutrient Management Research Database

Impacts of cover crops, tillage and abiotic factors on soil nitrogen (N) dynamics, greenhouse gas emissions, and microbiological functions were investigated in a vineyard in California’s Mediterranean climate. Treatments had been established in fall 2001 and were composed of two cover crops [Trios 102 (Triticale  Triosecale), (‘Trios’), Merced Rye (Secale cereale), (‘Rye’)] and cultivation (‘Cultivation’). Soils were sampled every 2–3 weeks from November 2005 to November 2006. Effects of season and treatment on potential nitrification and denitrification also were determined. Gravimetric water content (GWC) reflected winter and spring rainfall, and soil temperature generally did not differ among treatments. Microbial biomass N (MBN) typically was 2–3-fold greater in ‘Rye’ and ‘Trios’ than ‘Cultivation’ in winter and spring, but these differences among treatments disappeared in summer. Soil nitrate (NO3^-–N) was consistently greater in cultivated soils, with little temporal change in any treatment. In contrast, soil ammonium (NH4⁺–N) in cover crop treatments was 2–3-fold greater than ‘Cultivation’ in winter and spring, increasing in all treatments in summer after cover crops had been mowed and ‘Cultivation’ had been tilled. Significant multiple linear regressions of MBN on GWC, soil temperature, NH4⁺–N and NO3^-–N for all treatments indicated that GWC
significantly explained changes in MBN. Soil temperature also was significant for ‘Trios’
only, but its standard coefficient value was low, indicating its lesser importance in determining MBN. Despite a significant multiple linear regression of nitrous oxide (N2O) efflux on GWC, soil temperature, NH4⁺–N and NO3^-–N in ‘Trios’ only, no single variate explained the observed variation. However, increases in N2O were detected after both cultivation and increases in GWC from precipitation in winter, late spring, and fall. Mean daily N2O efflux was greater in cover crops, but annual N2O efflux was low as compared to fertilized and unfertilized annual cropping systems. Potential nitrification, N mineralization and denitrification were generally 2–4-fold greater in cover crop treatments than ‘Cultivation’. Thus, cover crops enhanced the soil’s capacity for supporting greater MBN, potential N mineralization, and the microbiological functions of nitrification and denitrification. Also, N dynamics appear to be more sensitive to changes in soil water content than temperature. We suggest that potential impacts of greater N2O emissions from cover crop soils be evaluated with reference to other benefits of cover cropping, such as increased soil organic matter content, improved microbiological activity, and N availability.

This study was conducted in a Chardonnay vineyard on Teleki 5C rootstock that was planted in 1996.  The cover crop and cultivation treatments were established four years before this study began.  

• 3 treatments were established, one with a Trios ("Trios") cover crop, one with a Merced Rye ("Rye") cover crop, and a cultivated ("Cultivation") treatment without a cover crop. There were two treatment plots of each treatment, in an experimental randomized complete block design.
• The study was conducted in late fall 2005 and late fall 2006, resulting in one season of cover crop and grapevine growth.
• The cover crop treatments were mowed in mid-April 2006 and residue was left on the soil surface.  The cultivated treatment was tilled about once every 2 months.
• Vegetation was sampled for above ground and root biomass and total N of plant tissue.
• Soil was analyzed for gravimetric water content (GWC), microbial biomass N (MBN), dissolved organic N (DON), denitrifying enzyme activity (DEA), and metrics of potential nitrification, total soil N and C.
• N2O emissions were measured using the static chamber method.

• Soil N content was was slightly higher in the Trios than in the Rye cover crop treatment.
• Cover crop treatments had greater microbial biomass N and potential N mineralization rates.
• Daily mean N20-N emissions were significantly greater in the cover crop treatments than in the cultivation treatment, although emissions were relatively low in comparison to other cropping systems.
• Potential for nitrification and denitrifying enzyme assays were greater in the cover crop treatments than in the cultivation treatment.