Quantifying global soil respiration (R ) and its response to temperature change are critical for predicting the turnover of terrestrial carbon stocks and their feedbacks to climate change. Currently, estimates of R range from 68 to 98 Pg C year , causing considerable uncertainty in the global carbon budget. We argue the source of this variability lies in the upscaling assumptions regarding the model format, data timescales, and precipitation component. To quantify the variability and constrain R , we developed R models using Random Forest and exponential models, and used different timescales (daily, monthly, and annual) of soil respiration (R ) and climate data to predict R . From the resulting R estimates (range = 66.62-100.72 Pg), we calculated variability associated with each assumption. Among model formats, using monthly R data rather than annual data decreased R by 7.43-9.46 Pg; however, R calculated from daily R data was only 1.83 Pg lower than the R from monthly data. Using mean annual precipitation and temperature data instead of monthly data caused +4.84 and -4.36 Pg C differences, respectively. If the timescale of R data is constant, R estimated by the first-order exponential (93.2 Pg) was greater than the Random Forest (78.76 Pg) or second-order exponential (76.18 Pg) estimates. These results highlight the importance of variation at subannual timescales for upscaling to R The results indicated R is lower than in recent papers and the current benchmark for land models (98 Pg C year ), and thus may change the predicted rates of terrestrial carbon turnover and the carbon to climate feedback as global temperatures rise.

Diet modification to reduce phosphorus (P) concentrations in manures has been developed in response to environmental concerns over P losses from animal agriculture to surface waters. We used USDA-NASS statistics on animal numbers and crop production to calculate county scale mass balances for manure P production, P removed in harvested portion of crops, and the potential effects of diet modification. Although spreading manure evenly over all crop acreage within a county is unlikely to occur, these calculations give a good indication as to the impact diet modification to reduce P can have at a regional or national scale. There was a high degree of regional variability in manure P surpluses (e.g., with the large crop acreages in the grain belt leading to large P offtake in crops preventing most P surpluses). In 89% of counties, there was a deficit of manure P relative to crop P removal; therefore there was a manure P surplus in 11% of counties. Diet modification decreased the percentage of states with a manure P surplus from 11 to 8%, a decrease of approximately 27%. Diet modification decreased the percentage of counties with the greatest surpluses of manure P (>30 kg ha(-1)) from 3% of all counties to 1%. Diet modification to decrease manure P is an important part of strategies to alleviate environmental concerns associated with surplus manure P in many areas, but additional strategies to deal with manure P surpluses are needed in some areas.

Cover crops are a major focus of conservation agriculture efforts because they can provide soil cover and increase nutrient availability after their mineralization in cropping systems. To evaluate the effect of residue type and placement on rate of decomposition and carbon (C) and nitrogen (N) mineralization, residues from two food crops, maize (Zea mays L.) and common bean (Phaseolus vulgaris L.), and two promising cover crops, sunn hemp (Crotalaria juncea L.) and sorghum sudangrass (Sorghum bicolor [L.] Moench x S. bicolor var. Sudanese [Piper] Stapf) were used in a litterbag study in the Central Plateau region of Haiti from May to September, 2013. Residues were placed in litterbags at a rate equivalent to 3.25 Mg residue ha−1 either on the soil surface or buried at 15 cm to represent a tilled and no-tillage system, respectively. Initial C:N ratios were: maize > common bean > sorghum sudangrass > sunn hemp. Highest residue mass loss rates and C and N mineralization generally occurred in the reverse order. Overall, surface-placed residues decomposed more slowly with 40 and 17 % of initial residue mass of surface and buried residues, respectively, remaining at 112 days. Carbon and N mineralization was higher when residues were buried. Net N mineralization of buried residues was 0.12, 0.07, 0.06, and 0.03 g N g residue−1 for sunn hemp, sorghum sudangrass, maize, and common bean, respectively over 112 days. To achieve the goal of increasing nutrient supply while maintaining year-round cover, a combination of grass and legume cover crops may be required with benefits increasing over multiple seasons.

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The objective of this study was to compare five methods with varying chemical approaches to the speciation of Al. The 8‐hydroxyquinoline (HQ) and ferron procedures were used to estimate the inorganic, monomeric forms of Al at reaction times of 15 and 30 s, respectively. Two other procedures, an ion exchange column procedure and a chelating resin procedure, were used primarily to measure organically bound forms of Al. These were compared with a F electrode technique which quantifies Al3+ activity using equilibrium thermodynamic calculations and measured values of F activity and total F. Stability constants are then used to calculate the speciation of inorganic forms of Al, and organically complexed forms are obtained by subtraction from total Al. Sample solutions containing 5 mmol m−3 F, and 7.9, 15.2, 44.5, and 80.4 mmol m−3 Al were synthesized, with and without addition of 1 mol m−3 (as dissolved organic C) purified fulvic acid extracted from the surface horizon of an Edneytown soil (Typic Hapludults). A soil solution extract was also obtained from the soil and analyzed. Excellent agreement (r = 0.999) between predicted and measured Al3+ was obtained for the electrode procedure in solutions without organic matter (OM). The kinetically reactive Al values obtained by the HQ procedure correlated very well with those of the F electrode procedure in predicting [Al3+] and toxic Al (Al3+ + AlOH2+ + Al(OH)+2), (r = 0.91 and 0.90, respectively), but overestimated these Al forms when the Al/organic C ratios were low. The kinetically reactive Al values obtained by the ferron procedure were greater than values obtained by the HQ and electrode procedures since ferron was also able to breakdown Al‐F complexes to a greater extent. The column and chelating resin procedures were able to separate organic and inorganic forms of Al only, thus speciation of inorganic complexes was not feasible. Organically bound Al calculated from the electrode procedure was generally lower but consistent with the values obtained by the chelating resin. The ion exchange column proceduure gave the lowest values of Al‐OM at the lowest Al/OM ratios, indicating that some degradation of Al‐OM complexes may occur during passage through the column. The F electrode procedure, though promising, should not be indiscriminately used over a wide range of pH, Al/F ratios, and DOC contents without more information on the effects of electrode interactions with organic solutes. Furthermore, the procedure is slow and requires the assumption of equilibrium conditions, which may seldom occur for Al in field conditions. It is, however, promising as a tool for the evaluation of other procedures under well controlled experimental conditions.