Can Cover Crops Improve Tomato Yield and Nitrogen Cycling in a Medium-Term Experiment?.

Cover crops (cc) are integrated into cropping systems for various reasons, but the longer-term impact of various cc on crop productivity has been largely overlooked. Therefore, two medium-term experiments (established in 2007 and 2008) as RCBD with 4 replications were used to assess the impacts of cc (no cover control (no-cc), oat, oilseed radish (OSR), winter cereal rye, and a mixture of OSR+Rye) on processing tomato yield and quality, and N cycling in 2015 and 2016. All fruit from the center 2m of two rows from each plot were harvested and graded. Three representative plants from the center two rows were sampled, separated into fruit and shoot, weighted dry, ground and a subsample analyzed by combustion for N concentration. At tomato harvest, a composite soil sample (6 cores, 3.5cm diameter) was taken from each plot. Soil was sieved (2 mm), 2M KCl extracted, ammonium-N and nitrate-N quantified and soil mineral N (SMN) expressed as content based on bulk density (0-60 cm). In 2015, oat had higher marketable yield than rye (91.4 vs. 76.6±3.90 Mg ha^-1) (P<0.05); no differences with other cc were detected. In 2016, OSR had the highest marketable yield (125±5.84 Mg ha^-1) with the trend of OSR≥OSR+Rye>no-cc=oat=rye. Tomato fruit quality (color, soluble solids, pH) was not influenced by cc in both years. In both years, the no-cc treatment had the highest SMN (169±4.31 kg N ha^-1) followed by OSR=oat=OSR+Rye>rye. Tomato plant N content (fruit+shoot, 168±47.2 kg N ha^-1) was not significantly affected by cc in both years. Hence, the high tomato yield with no difference in the plant N content along with high residual N suggests that the observed cc effect on crop yield was due to the factors other than N dynamics, (perhaps soil health, water dynamics, soil microbial communities etc.); thus, leading to future research.