Surface application of urea to pine forests may lead to ammonia (NH3) loss. It is generally believed that rainfall received soon after urea application will wash the urea and its hydrolysis products into the soil and stop NH3 loss, but quantitative data are lacking, especially for the forest environment. The objective of this study was to quantify the effect of rainfall on loss of NH3 when received at different times following urea application. Four field studies were performed in a midrotation loblolly pine (Pinus taeda L.) plantation, where NH3 volatilization chambers were fertilized with 200 kg ha−1 N and NH3 losses were measured for either 29 or 58 d. In a complementary lab study, both NH3 loss and movement of fertilizer N into the soil were measured following simulated rain. Loss of NH3 from urea was either increased or not affected by simulated rainfall applied after the urea granules were dissolved by dew. Increased NH3 loss due to simulated rainfall was attributed to inefficient downward leaching of urea and increased water content, which is known to increase the rate of urea hydrolysis. In contrast, simulated rainfall applied immediately after urea application reduced NH3 losses to <1% of the applied urea. Our results show that unless rain occurs before urea is dissolved by morning dew, it may not be effective at leaching urea into the soil and reducing NH3 losses. Further research should be conducted to elucidate the mechanism of urea retention by the O horizon in pine forests.
The use of urea fertilizers in grasslands is likely to increase in areas with concentrated animal feeding operations as restrictions on manure applications are implemented. Concerns have been raised about the economic and environmental impacts of NH3 loss from these urea fertilizers. This study evaluated NH3 losses from Nitamin (a urea polymer), urea–NH4NO3 (UAN), and granular urea applied to tall fescue (Festuca arundinacea Schreb.) plots at 50 kg N ha−1 in fall and spring for 2 yr. Fertilizers were applied to circular plots (30‐m diameter) and NH3 loss was measured by the modified passive flux method for 69 to 120 d after application. In a separate laboratory study, Nitamin, UAN, and urea were surfaced applied to fescue thatch at 100 kg N ha−1 and treatments were incubated at 24°C and 90% relative humidity for 31 d. In fall applications, urea lost more NH3 (19% in 2004, 46% in 2005) than UAN or Nitamin, which were not different from each other (6% in 2004, 34% in 2005). In contrast, there were no differences among fertilizers in spring applications, with average losses of 13% in 2005 and 17% in 2006. In the laboratory study, urea lost significantly more NH3 (24%) than UAN or Nitamin, which were not different from each other (average 9% loss). These results indicate that Nitamin and UAN undergo similar NH3 losses, and that both fertilizers may lose less NH3 than urea under conditions favorable to volatilization.
Core Ideas The model provided good estimates of N mineralized when linked to climate data. Modifications to rate constants and some parameters led to good model fit. The model predictions in the validation were accurate for incorporated residues. Cover crops can provide substantial quantities of N for subsequent crops, but estimating the amount of N that will be mineralized from residues is challenging. Complex interactions of residue chemistry with soil temperature and soil water content affect N mineralization during residue decomposition. A simulation model can describe these interactions and provide estimates of N mineralized if specific soil water and temperature data are available. Our objectives are (i) to describe a web‐based N mineralization model and its operation, (ii) to calibrate the model with results from published N mineralization studies, and (iii) to validate it using field studies investigating decomposition of surface‐applied or incorporated crimson clover (Trifolium incarnatum L.) or rye (Secale cereale L.) residues over 3 yr. Inputs required by the model include residue N, nonstructural carbohydrates, cellulose + hemi‐cellulose, and lignin contents, as well as 5‐yr average values of daily soil temperature and soil water content from a user‐selected weather station. The model was successfully calibrated with published data from eight laboratory and field studies and was validated with data from field studies that used soil cores with cover crop residues. Simulated values of N mineralized were acceptable for incorporated residues but tended to overpredict N mineralized from surface residues because soil temperature and water content are not good drivers to simulate N mineralization from residues on the soil surface. Additional research is needed to develop algorithms to estimate temperature and water content/water potential of surface residues so they can be used as driver variables for the model.
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