Since annual fluctuations in climate are unlikely to have the same effect on ecosystems as permanent changes, our estimates fail to account for these effects too. Second as its name suggests, global climate change will affect agricultural production around the globe. It may be reasonable to assume that this will alter the long run costs of production and this would cause changes in relative prices. Since our estimates use annual fluctuations in climate and are adjusted for state by year fixed effects, they do not account for this possibility. It is noteworthy that the hedonic approach is unable to account for such changes either because the land value-climate gradient is estimated over the existing set of prices. Third, there are a complex system of government programs that affect agricultural profits and land values by affecting farmers’ decisions about which crops to plant, the amount of land to use, and the level of production . Our estimates would likely differ if they were estimated with an alternative set of subsidy policies in force. It is notable that this caveat also applies to the hedonic method. Fourth, our measure of agricultural profits differs from an ideal one in some important respects. In particular, interest payments are the only measure of the rental cost of capital in the Censuses. This measure understates the cost of capital by not accounting for the opportunity cost of the portion of the capital stock that is not leveraged. Further,plastic gutter our measure of agricultural profits does not account for labor costs that are not compensated with wages . Finally, we discuss two caveats to our approach that would lead to downward biased estimates of the impact of global warming, relative to an ideal measure.

First as we have emphasized, our approach does not allow for the full set of adaptations available to farmers. We again note that this causes the estimates in Table 8 to be biased downwards from a measure that allows for the full range of compensatory behavior. The direction of the bias can be signed, because farmers will only undertake these adaptations if the present discounted value of the benefits are greater than the costs. Second, elevated carbon dioxide concentrations are known to increase the yield per planted acre for many plants . Since higher CO2 concentrations are thought to be a primary cause of climate change, it may be reasonable to assume that climate change will be associated with higher yields per acre. The approach proposed in this paper does not account for this “fertilizing” effect of increased CO2 concentrations. In intensive agriculture, irrigation and fertilization can lead to rapid increases in soil salinity and acidity . Acidity and salinity are known to have profound stressful effects on nutrient cycling . Practices such as fertigation through drip systems, which aim to improve input use efficiency by targeting water or nutrients to the plant rooting zone, can exacerbate the magnitude and heterogeneity of acidity or salinity stresses by concentrating them in smaller soil volumes . The use of subsurface drip irrigation has expanded very rapidly in recent decades. For example, in California’s processing tomato industry, the proportion of growers using drip irrigation rose from 0% in 1987 to 85% in 2011 . While SDI generally increases yields and water use efficiency, it can also cause rapid acidification under intensive processing tomato production . A preliminary field study suggests that SDI may carry trade offs for microbial nutrient cycling in ways that are not yet fully understood . In managed systems, acidity and salinity stresses can generally be corrected rapidly through liming or leaching.

However, most work examining the impact of acidity and salinity on nutrient cycling has been done in static systems in which soils and microbial communities have had decades or centuries to adapt. Few studies have tested how acidity and salinity impact nutrient cycling under the fluctuating conditions which characterize intensive agriculture, and the recovery of those processes after the stress is relieved . Additionally, while the effects of acid and salinity stress on carbon cycle processes have been well documented, the effects on nitrogen cycling are unclear. Chemical stresses have been shown to reduce microbial growth, respiration, and C use efficiency . Nitrification is also consistently inhibited . However, the effects of acidity and salinity on N mineralization are mixed. Some have observed that net N mineralization at low pH is reduced and that lime application increases N mineralization, at least temporarily . Conversely, others have found no clear relationship between pH and N mineralization , very fast ammonification , and gross mineralization rates sometimes occur in the most acid soils in a gradient. As coupled cycling of C and N contributes to the efficiency of agricultural systems , a better understanding of their joint reaction to chemical stresses is a necessary component of sustainable agricultural intensification. In an agricultural context, compost is a potential tool for buffering the impacts of chemical stress on microbial functions. Compost has been reported to increase proton consumption capacity and reduce aluminum toxicity in acid soils, improving crop growth . Additionally, results from pH and salinity gradients in natural studies and long-term agricultural sites suggest that the effects of stress on microbial community structure and function, and the length of their recovery time if the stress is relieved, may be reduced in soils with a higher soil organic matter content or where a substrate is added . It seems possible therefore that compost addition could buffer the effects of a stressful management-induced change on microbial N and C cycling and allow for a faster recovery. Compost use is currently incentivized due to its potential benefits for climate resilience . However, its potential for improving resilience to chemical stresses has yet to be fully explored. The first objective of this study was to measure how the microbial biomass and several metrics of N and C cycling activity were affected by the imposition, persistence, and partial alleviation of acidity and salinity stresses in combination. The second objective was to test whether compost application at an agronomically realistic rate moderated these effects. To this end we performed a greenhouse experiment in which chemical stress was imposed by mixing powdered elemental sulfur into an agricultural soil with or without compost. Sulfur was chosen as a stressor because it does not add N, has multiple agricultural uses,blueberry container can be uniformly mixed with soil, and rapidly increases both acidity and salinity . We hypothesized that 1) Microbial biomass and metrics of microbial N and C cycling activity would initially be strongly repressed by Sinduced stress, 2) All functions and pools would recover to varying extents following alleviation, and 3) The presence of compost would buffer the stress response result in a faster and more complete recovery. The present study is part of a larger project that investigates the effects of induced stress and soil health promoting practices on soil microorganisms and crops. We aimed to measure how several N and C cycle pools and processes changed under a strong, short-term stress, how functions could recover after partial stress alleviation, and how those stress response dynamics were moderated by compost addition. We observed that patterns of initial response, adaptation, and recovery differed strongly between different N and C cycling functions. Four clear responses to S-induced stress were observed: an almost complete inhibition of nitrifier activity, a long-lasting increase in DSOC0 and DSON0 pools, an apparent decoupling of C and net N mineralization responses, and a pronounced, consistent increase in net N mineralization from a labile residue addition in the compost-amended stressed soils.

A summary comparison of the effects of stress on selected C and N cycle processes is given in Fig. 5. The minimum pH reached in the acidified soil was similar to minimum pH observed in soils from this region following the application of high rates of NH4 fertilizers . Below pH 4.5, all bacterial and fungal growth were observed to be heavily impacted in a long-term agricultural pH gradient, even when stimulated with substrate additions .In our study, soluble Al began to increase exponentially below pH 5.5. While the Almono concentrations were low, the significant negative correlation with MBC during the period of pH stress suggests Al toxicity may have been a stressor . While inter-study salinity comparisons are difficult due to strong effects of moisture, texture, and the method used to measure electrical conductivity, our salinity values are at the lower end of those observed to impact microbial growth and activity . Therefore, soil pH was likely a stronger stress than salinity in our study. Although humic substances extracted from composts have been observed to reduce free Al and green waste composts can consume protons in acid soils , our green waste compost did not have any discernible effect on Al or pH during the time scale of this experiment. Similarly, McTee et al. found that compost did not raise pH or improve plant growth in soil from a skeet shooting range heavily contaminated with S.The strongest functional change due to S addition was an almost complete absence of nitrification from added residues. This is in line with several long-term studies which consistently observed lower nitrification in both acid and saline soils . Nitrification has also been observed to be limited by free Al . At low pHs, the direct mechanism is thought to be a lack of NH3 substrate for ammonia monooxygenase, the main enzyme responsible for NH3 oxidation . However, the fact that liming did not rapidly increase nitrification capacity suggests that the inhibition at this point was due more to the loss or slow recovery of nitrifier community members than the lack of substrate or acid cation toxicity. Nitrification is a multi-step process, carried out by different groups of rather specialized organisms, and successful nitrification depends on their coupled function . When naturally acid soils are limed, organic matter in the soil solution usually increases as organic matter functional groups are deprotonated and become less attracted to negatively charged mineral surfaces . The high DSOC0 and DSON0 in the S+ treatments show that the reverse does not necessarily hold when neutral soils are acidified. Our findings parallel those of Kemmitt et al. for long-term agricultural pH gradients. These authors also observed that dissolved organic C and N were negatively correlated with microbial biomass and activity and closely positively correlated with acidity and exchangeable Al. Decreased microbial metabolic capacity due to acid cation toxicity could help explain the high DSOC0 and DSON0 in the acidified soils . Complexation with Al may also reduce organic matter’s susceptibility to microbial attack ; however, the very low proportion of complexed soluble Al at low pH suggests this mechanism was not relevant. It is also possible that the acid stress increased soluble organic matter supply through the activity of soil enzymes. While we observed that potential enzyme activities were reduced in the S+ treatments compared to the S- treatments under standardized analytical conditions, actual activity may have been higher due to the low pH optima of both BG0 and NAG0 . As predicted in our first hypothesis, stress initially strongly reduced microbial biomass, respiration, and N mineralization. However, the more marked decline in respiration than biomass at 21 DAI was unexpected, as stress generally increases the respiration per unit biomass . A joint inhibition of respiration and growth such as was observed at 21 DAI may be a sign of a poorly adapted community dealing with a specific toxicity which induces widespread death or dormancy . The low N mineralization which was also observed at this date is consistent with a community undergoing severe metabolic limitation . Indeed, the slightly but consistently negative values for Nmin-Soil in the S+ treatments and reduced Mineral N0 at 21 DAI suggest the small microbial community may have been immobilizing N, as significant losses by denitrification and volatilization are unlikely in acidic, low organic matter, aerobic soils . Our finding that microorganisms initially depleted the mineral N pool while the DSON0 pool increased suggests preferential uptake of mineral N. However, after a longer time of equilibration with the stress, with no change in pH or DSON0 and an actual decrease in NAG0 activity, NminSoil increased in the S+ treatments to the point that it exceeded that of the S- treatments.