Joaquin Sanabria

Precipitation Regulates the Response of Net Ecosystem CO2 Exchange to Environmental Variation on United States Rangelands

Abstract Rangelands occupy 50% of Earths land surface and thus are important in the terrestrial carbon (C) cycle. For rangelands and other terrestrial ecosystems, the balance between photosynthetic uptake of carbon dioxide (CO2) and CO2 loss to respiration varies among years in response to interannual variation in the environment. Variability in CO2 exchange results from interannual differences in 1) environmental variables at a given point in the annual cycle (direct effects of the environment) and in 2) the response of fluxes to a given change in the environment because of interannual changes in biological factors that regulate photosynthesis and respiration (functional change). Functional change is calculated as the contribution of among-year differences in slopes of flux-environment relationships to the total variance in fluxes explained by the environment. Functional change complicates environmental-based predictions of CO2 exchange, yet its causes and contribution to flux variability remain poorly defined. We determine contributions of functional change and direct effects of the environment to interannual variation in net ecosystem exchange of CO2 (NEE) of eight rangeland ecosystems in the western United States (58 site-years of data). We predicted that 1) functional change is correlated with interannual change in precipitation on each rangeland and 2) the contribution of functional change to variance in NEE increases among rangelands as mean precipitation increases. Functional change explained 10–40% of the variance in NEE and accounted for more than twice the variance in fluxes of direct effects of environmental variability for six of the eight ecosystems. Functional change was associated with interannual variation in precipitation on most rangelands but, contrary to prediction, contributed proportionally more to variance in NEE on arid than more mesic ecosystems. Results indicate that we must account for the influence of precipitation on flux-environment relationships if we are to distinguish environmental from management effects on rangeland C balance.

Nitrogen uptake kinetics of key staple cereal crops in different agro-ecological regions of the world

ABSTRACT The study was undertaken to assess the effect of environmental, management, and stress factors on nitrogen uptake patterns through the crops’ growth cycle and to associate temporal patterns of N uptake with biomass and grain yields. Existing complete experimental data, provided by several institutional databases and through an extensive literature review, were utilized together with crop simulation models (CSMs) to synthesize yield and N uptake profiles of the key staple cereal crops in selected agro-ecologies. Approximately 465 observations were identified for combined maize grain yield and plant N uptake: 156 for rice and 254 for wheat. The Decision Support System for Agrotechnology Transfer (DSSAT), which comprises CSMs and data that integrate capabilities on soils, daily weather, crops, and management, was used in combination with field information to first validate the CERES-maize, -rice, and -wheat models. The most noteworthy results from synthesis of the data set for the three key cereals were as follows: (a) N uptake continued to increase with time until physiological maturity with adequate N supply; (b) significant effect of soil N status on N uptake kinetics was observed at zero N; (c) N uptake profile was also influenced by the planting date, with the summer planting showing higher uptake than other planting dates; (d) field methods of N application influenced N uptake kinetics: a one-time injected or subsurface-applied urea continued to provide an adequate amount of N throughout the crop growth phase that was comparable or even higher than with broadcasting multiple splits; (e) N uptake was also dependent on crop cultivars, including stages of vegetative and reproductive phases, with shorter vegetative and longer reproductive phases showing continuous N uptake and lesser dependence on N remobilization; and (f) predictions suggested that modest changes in ambient temperature and atmospheric carbon dioxide (CO2) concentrations would not significantly alter the N uptake kinetics, with the uptake rate expected to increase under future climate change scenarios. The combined data suggest that no one N uptake kinetic pattern fits all crops under all environments and management practices.

Changes of Soil Microbial Population and Structure Under Short-term Application of an Organically Enhanced Nitrogen Fertilizer

Abstract Interest in the use of alternate fertilizers has increased during recent years to improve soil productivity. An organically enhanced N fertilizer, containing 14.9% N, 4.3% P2O5, 18.1% S, 0.6% Fe, and 8% organic C, and is produced from a sterilized organic additive extracted from municipal wastewater biosolids and chemical fertilizers was evaluated for its effects on soil microbial populations and abundances in 0- to 15-cm depth of of two silt loam soils located at Jackson and Grand Junction, Tennessee. This treatment was compared to conventional N fertilizers and zero N control under nonirrigated corn (Zea mays L.) from 2011 to 2013. Three N-applied treatments (organically enhanced N fertilizer, ammonium sulfate, urea/NPKZn briquette) at 128/170 kg ha−1 and the zero N control were imposed at each location. The organically enhanced N fertilizer decreased the relative abundance of arbuscular mycorrhizal fungi but increased that of general microbes relative to the zero N control and increased that of general microbes compared with NPKZn briquette 4 to 7 months after their applications at an N rate of 128 kg ha−1 for corn within 2 years of experimentation on a relatively infertile soil with low organic matter. Soil general microbes and arbuscular mycorrhizal fungi were the two sensitive indicators of soil microbial structure response to fertilization. However, effects of the organically enhanced N fertilizer on soil microbial populations were not noticeable after corn harvest. In conclusion, application of the organically enhanced N fertilizer has noticeable influence on soil microbial structure/abundance but not on populations on relatively infertile soils with low organic matter from a short-term perspective.