The new Caribbean Nitrogen Index to assess nitrogen dynamics in vegetable production systems in southwestern Puerto Rico

Authors: OLIVERAS-BERROCALES, M - Natural Resources Conservation Service (NRCS, USDA); SOTOMAYER-RAMIREZ, DAVID - University Of Puerto Rico; Delgado, Jorge; PEREZ-ALEGRIA, LUIS - University Of Puerto Rico

Submitted to: International Soil and Water Conservation Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2017
Publication Date: 2/15/2017
Citation: Oliveras-Berrocales, M., Sotomayer-Ramirez, D., Delgado, J.A., Perez-Alegria, L.R. 2017. The new Caribbean Nitrogen Index to assess nitrogen dynamics in vegetable production systems in southwestern Puerto Rico. International Soil and Water Conservation Research. 5:69-75. doi:10.1016/j.iswcr.2017.01.001.

Interpretive Summary: Residual soil nitrate simulated using the N Index was significantly correlated (r=0.88) with measured soil nitrate (Figure 2; P<0.05), with an RMSE of 17. Thus, the model seemed accurate in predicting residual soil N after crop harvest in onion, pumpkin, pepper, and tomato cropping systems. The values simulated by the N Index for aboveground crop (fruit + plant biomass) N uptake were significantly correlated (r=0.99) with those measured ( P<0.05), with an RMSE of 4.5. Thus, the model was quite accurate in predicting crop N uptake. The mean nutrient extraction per unit of crop yield in onion was 3.1, 3.0, 3.1 and 3.2 kg N/ton for the 140, 189, 196 and 253 kg N/ha fertilizer treatments, respectively. In pumpkin these were 4.5, 7.0 and 6.0 kg N/ton for the 112, 196 and 281 kg N/ha fertilizer treatments, respectively. In pepper and tomato, these were 2.7 and 4.1 kg N/ton, respectively, with a 275 kg N/ha fertilizer application. The simulated N losses for the loss pathways ranged between 1 to 4 kg N/ha for volatilization, 18 to 65 kg N/ha for denitrification, and 155 to 779 Kg N/ha for lixiviation. According to N Index simulations, less N was lost due to volatilization; however, lixiviation was the principal pathway of N loss to the environment. This could be due to the shallow rooting system of vegetables, the continuous use of irrigation, and the amount of rain received. The mean NUE predicted by the N Index was 26% (±8). Our mean NUE values are similar to those reported by De Paz et al. (2009) of 27% (±5) for various vegetable crops (tomatoes, onions, eggplants, lettuce, spinach, and carrots). Our low NUE is apparently due to the high N lixiviation simulated, which was on average 403 kg N/ha, for all crops. A similar response was reported by Delgado (1998; 2001), who found a correlation between root depth and N lixiviation. Delgado (1998; 2001) reported that more N is lost by lixiviation when shallow-root crops such as vegetables are used. Vegetable crops have a high potential for N losses via lixiviation due to them having a shallow rooting system compared to other crops like fruit trees such as citrus. This study demonstrates that the Caribbean N Index Caribbean is capable of assessing N dynamics in vegetable cropping systems in the LVAR in Puerto Rico. It also shows that the model is capable of simulating the effects of different management practices on N loss pathways.

Technical Abstract: Nutrient loss from agricultural fields is one of the main factors influencing surface- and ground-water quality. Typical fertilizer nitrogen (N) consumption rates in vegetable production systems and horticultural crops in Puerto Rico, fluctuate between 112 to 253 kg N/ha. Nitrogen use efficiency of vegetables crops is low, increasing the potential for nitrogen losses and high residual soil nitrate contents. Quantification of residual soil N and N losses to the environment can be a difficult task. Simulation models such as the N Index (USDA-ARS) can be used to identify the relative magnitude of varying N-loss pathways and to identify best management practices. Field studies were conducted to quantify residual soil N, crop N uptake, and validate the N Index in onion, tropical pumpkin, and tomato production systems in the Lajas Valley in southwestern Puerto Rico. Relationships between observed and simulated values were determined to examine the appropriateness of the model for evaluating N losses. There was a good correlation between observed and predicted values for residual soil N (r = 0.88) and crop N uptake (r = 0.99) (p< 0.01). In the production systems evaluated, the N losses ranged between 1 and 4 kg N/ha for volatilization, 18 and 46 kg N/ha for denitrification, 155 and 779 kg N/ha for lixiviation and between 64 and 401 kg N/ha for residual soil N. The N use efficiency ranged between 17 and 39%. The results obtained showed that the N Index tool can be a useful tool for evaluating N transformations in vegetable production systems of Puerto Rico.