Effect of grafting and cover crop quality on nitrogen uptake from different pools by drip irrigated heirloom tomatoes in an open field trial

Fresh-market tomatoes (Lycopersicon esculentum Mill.) are increasingly produced with subsurface drip irrigation in California, as growers adapt to decreased water availability. Drip irrigation improves water use efficiency, but may reduce the soil volume where conditions are optimal for microbial and root activity. This poses complications for organic growers, whose cropping systems are dependent on microorganisms to convert the nitrogen (N) from organic sources into plant-available N. To investigate grafting as a potential practice to increase access to N, grafted and non-grafted "Brandywine" heirloom tomatoes were grown with drip irrigation in a two-year field trial. The three rootstocks were "Maxifort," "Fortamino," and "DRO138TX." Prior to each growing season, a legume cover crop mix was grown and labeled with N-15 ammonium sulfate to track tomato N uptake from cover crop residues. There were no significant differences in yield, N uptake and N partitioning in the plants due to rootstock treatments. Leaf analyses suggested that the tomato plants were not N deficient in either year. However, there was a significant difference in total N uptake from cover crop residues between years. This was caused by large differences in N concentration in the cover crop. Plants responded to a lower N availability by producing less vine biomass, while producing the same fruit yield. A budget revealed that the tomato plants accessed a considerable amount of N from the soil below 60 cm of the profile to cover their demand.