Assessing the Response of Tomato Yield, Fruit Composition, Nitrogen Absorption, and Soil Nitrogen Fractions to Different Fertilization Management Strategies in the Greenhouse

A greenhouse field experiment involving tomato (Solanum lycopersicum) was performed using different nitrogen (N) management regimes: sole application of differing rates of chemical N fertilizer (SC) (SC treatments: N0, N1, N2, and N3) and combined application of manure and chemical N fertilizer (MC) (MC treatments: MN0, MN1, MN2, and MN3). These were used to understand the relationship between comprehensive fruit composition, yield, and N fractions (soil mineral N; soil soluble organic N; soil microbial biomass N, and soil fixed ammonium) under greenhouse conditions. The results showed that the MC treatments significantly increased vitamin C and soluble sugar content compared with SC treatments. In addition, the MN2 treatment produced a high yield and had a positive effect on fruit composition. The N3 (563 kg N/ha) and MN3 (796 kg N/ha) treatments resulted in a high loss of N below the root zone (0–30 cm), consequently reducing N use efficiency. Soil mineral N, soil soluble organic N, and soil fixed ammonium tended to be higher during the first fruiting period, whereas soil microbial biomass N tended to be higher during the second fruiting period. MC treatments significantly increased the N fraction in the 0- to 30-cm soil layer; N fractions tended to be higher with the MN2 treatment. According to an optimum regression equation, soil fixed ammonium during the first fruiting period and soil microbial biomass N during the second fruiting period had a more significant influence on tomato yield and fruit composition. Overall, application MC at an appropriate rate (MN2: 608 kg N/ha) is a promising approach to achieving high yields and optimum taste, and it offers a more sustainable fertilizer management strategy compared with chemical N fertilization.

Influence of legumes on N cycling in a heathland in northwest Spain

Nitrogen availability frequently limits plant growth in natural ecosystems. N-fixers should have a substantial competitive advantage in N-limited systems, and as a byproduct of their activity they should increase the quantity and availability of N in the system as a whole. However, this effect has rarely been quantified in natural ecosystems. Heathlands in northwest Spain are frequently occupied by legume scrubs. We tested whether the presence of these legumes affected the N cycle in these communities. Specifically, we addressed the following questions: is nitrogen availability higher beneath legume canopies than beneath non-legume canopies? Is soil microbial biomass acting as a sink of extra N mineralized beneath legume canopies? Does the presence of legume scrubs change the soil pools of labile N and P? Is N plant uptake different under N-fixer scrubs than under non-N-fixer scrubs? To answer these questions, we sampled soil beneath the canopy of randomly selected individuals of Erica umbellata, Ulex gallii, and Genista tridentata twice during the growing season. Soil samples were analyzed for organic matter, NH4-N, NO3-N, DON, PO4-P, N mineralization and nitrification rates, and soil microbial biomass-N. In addition, we estimated N uptake by plants and the N concentration in green tissue to compare internal N cycles between legume and non-legume scrubs. Nitrification rates, DON (dissolved organic nitrogen), soil NO3 concentration, and N uptake were significantly higher beneath legume canopies. However, soil microbial biomass-N and extractable-P were significantly lower under legumes. Our results showed that the presence of legume scrubs modify the size of N pools and the dominant form of available N for plants, increasing spatial heterogeneity in mixed stands.