Estimating Available Nitrogen in Central Valley Soils

Predicting nitrogen mineralization has been the focus of many agricultural researchers for decades. This project was designed to investigate the soil properties that influence potential nitrogen mineralization in the northern Central Valley of California. A total of seventeen sites featuring annual cropping systems with a variety of common crops such as corn (Zea mays), watermelon (Citrullus lanatas), safflower (Carthamus tinctorius), sunflower (Helianthus annuus), sorghum (Sorghum bicolor), processing tomatoes (Solanum lycopersicum), and cotton (Gossypium hirsutum) were included, spanning from Colusa to Tulare Counties. We studied nitrogen mineralization using both field trials as well as laboratory incubations of undisturbed soil cores, in conjunction with continuous soil sampling of those sites every 5-6 weeks throughout the growing season to capture seasonal nitrogen turnover. Those soils were characterized and the results statistically analyzed, and a model was developed that determined particulate organic nitrogen and silt content as the best predictors of nitrogen mineralization in the undisturbed soil cores (adjusted-R2 = 0.65, P < 0.05). Over 70% of the variability in seasonal nitrogen mineralization at optimal soil moisture was described by only particulate organic nitrogen, while soil moisture was confirmed as a major controlling variable in determining nitrogen mineralization. Furthermore, even at the lowest soil moisture contents, nitrogen mineralization continued. Two separate smaller studies were also performed to determine crop residue effects on nitrogen availability. One included a field trial using seven different crop residues followed by a laboratory incubation of undisturbed soil cores, and the other included a laboratory incubation with two different residues at varying residue and soil moisture content. Seven different common crop residues (namely corn (Zea mays), watermelon (Citrullus lanatas), safflower (Carthamus tinctorius), sunflower (Helianthus annuus), sorghum (Sorghum bicolor), processing tomatoes (Solanum lycopersicum), and cotton (Gossypium hirsutum) were incorporated in the field trial. By the end of the incubation, cotton residues resulted in the greatest increase in soil mineral N, and sunflower residues resulted in the greatest nitrogen immobilization when compared to the control without residue. The difference between the two treatments was 13.7 mg N kg-1 OD soil, or approximately 28.4 kg N ha-1 in the top 15 cm of the soil. Finally, we performed two 10-week laboratory incubations investigating soil and residue moisture’s effect on nitrogen mineralization using processing tomato and broccoli residues. At the beginning of both incubations, residue moisture had a significant effect on nitrogen mineralization, however, by the end of both incubations, residue moisture was no longer significant, while soil moisture became increasingly important (P < 0.05). During the 10-week incubation, 21.6% and 50% of the total nitrogen in tomato and broccoli residues was mineralized, respectively. Differences are likely due to the residue’s carbon:nitrogen ratios and lignin content. The results of these studies will be used by growers in the region to improve upon their fertilizer management practices and make informed decisions based on cropping history and soil properties