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Transient responses to bio-available fractions may have occurred prior to our first measurement

As attested by the reduced MBC0 but increased CminSoil observed at 42 DAI, the community at this point appeared to be slower growing but better able to metabolize organic matter in an acid environment . A high rate of respiration to growth is a well-documented characteristic of stress adapted microbial communities . Stoichiometrically, a lower community metabolic efficiency could also help explain the observed increase in Nmin-Soil . Significant shifts in community tolerance to acidity have been observed within 36 d , making it plausible that some shift in acid tolerance could be observable within the 42 d under strong acid stress. The tendency towards higher net N mineralization in the S+ soils than the S- soils was much more pronounced with the addition of legume residues. The fact that both C and N mineralization responded so much more strongly to residue additions in the S+ than S- soils despite the former’s higher levels of soluble organic matter suggests that the mineralization pulses were not due to relief of substrate limitation. Since legume residues can complex with Al and reduce its activity , as well as temporarily consume protons through decarboxylation and ammonification of soluble organic acid anions , it is possible that the residues stimulated activity by relieving acid cation toxicity which had been limiting metabolism . As decarboxylation and ammonification produce CO2 and NH4 +, respectively , such detoxification products could also have contributed to the observed mineralization pulses. Liming produced mixed effects on C and N cycling processes. The most obvious effect of liming was a very large CO2 pulse from the unamended soil,growing blueberries in containers far exceeding the DSOC0 pool. It is likely that at least part of this was abiotic, issuing from the decomposition of carbonic acid from the liming reaction to CO2 .

While it is not possible to separate biotically and abiotically generated CO2, the fact that additional respiration due to residues was remarkably similar before and after leaching suggests that liming did not increase the capacity for respiration when adequate substrate was present. Contrary to our hypothesis, MBC0 and potential BG0 activity showed no signs of recovering after alleviation. However, the tendency towards higher MBC-Res with equivalent CO2-Res suggests that the community that grew in response to residue additions after liming was more efficient than that which responded at 42 DAI. The high Nmin-Res during stress and decline after liming suggests that high net N mineralization in response to substrate addition was caused by an inefficient community whose growth was limited by the adaptations required to survive in a stressful environment. Our results are in line with several studies which found that net N mineralization was not inhibited by salinity and acidity to the same extent as C mineralization and nitrification . Indeed, both net and gross N mineralization have sometimes been observed to be highest in the most acid soils within an experimental gradient . Similarly, a 400% increase in net N mineralization from vetch residues was measured in response to Al additions, despite reductions in C mineralization and MBC . The most direct explanation for this effect is that immobilization is slower than mineralization at low pH ; however, this does not always seem to be the case . The fact that the increase in MBC-Res after liming was not proportional to the decline in Nmin-Res suggests that reduced immobilization did not entirely explain the mineralization pulse. Other hypothesized mechanisms include increased losses by denitrification or volatilization as pH increases . Contrary to our hypothesis, compost had no effect on stress response and did not affect most indicators, regardless of S treatment. Compost is generally a stable, microbially processed product, rich in condensed, high molecular weight compounds, phenols and lignin and depleted in energetic compounds such as sugars .

This may explain why it did not have a measurable effect on microbial growth or most activity within our experimental time frame.Conversely, compost strongly and consistently increased Nmin-Res across all three sampling dates. As an increased Nmin-Res was likely a stress response, compost appears to have exacerbated the effects of stress, rather than buffering it as hypothesized. This paradoxical result could be explained if the increased net N mineralization under stress was partly due to a community shift towards one with less need for N relative to C. Fungi tend to have a higher C:N ratio than bacteria and are thought to generally be more acid tolerant . Rapid fungal but not bacterial growth rates have been observed within days of a labile residue addition to acid soils , and high fungal: bacterial ratios have been observed in experimentally acidified grassland soils . Since fungi generally have a wider C:N ratio than bacteria, they immobilize less N per unit C fixed. A faster fungal than bacterial growth response to residue additions could help explain why Nmin-Res values in the S+ treatments were on average more than double those in the S- treatments. The presence of a carbon source such as higher SOM or crop inputs often improves community stress adaptation . Adding compost could have facilitated that stress-induced community shift, such that the organisms which responded to residue additions in the C + S+ soil needed less N than their counterparts in the C-S+ soil . Strong community shifts are not necessarily evident in respiration measurements due to functional redundancy. For example, at the Hoosfield acid strip, fungal growth rates increased 30- fold as pH declined from 8.3 to 4.5, while respiration changed by less than one third . A compost-facilitated shift towards a less N-retentive community would be in line with two recent studies which observed that soils which were fungally dominated due to acid stress tended to use substrate less efficiently . This work presents the first data on the ability of a compost to moderate the effects of acid stress on nutrient cycling.

Green waste compost was chosen for this experiment, as it is typical of the type of compost the production and use of which is predicted to rapidly expand in California . It is important to note that these results may not be typical of all compost types. For example, a strong liming effect has been observed when poultry manure from layer hens was applied to an acid soil, likely due to the calcium carbonate in the feed . However, the strong and unexplained effect on N cycling suggests that further investigation with additional soil and compost types should be pursued. In particular, compost effect on microbial community structure under chemical stress should be investigated further. Additionally, the use of isotopically labeled residues would allow for mechanistic exploration of mineralization and immobilization dynamics. California’s agricultural sector critically affects both the national food supply and regional water resources. California has the largest agricultural sector in the country, producing two thirds of the fruits and nuts in the United States and approximately one third of its vegetables . California’s crop supply is also significant to the United States in that many crops grown in the state, such as almonds, garlic, olives, raisin grapes, pistachios, and walnuts,square pots are exclusively produced there . However, while California’s more than 400 commodities are central to US food supplies, they also necessitate high water inputs. High crop production and a semi-arid climate result in agricultural needs using over eighty percent of the state’s managed water supply . This reliance on irrigated inputs means that yearly crop prices and food supplies in the United States are susceptible to changes in the available water supply of California and impacted by local water management decisions . As California’s water supply becomes increasingly unpredictable due to changes in climate, this interconnection of food and water supplies at local to national scales is ever more important to understand. California’s highly variable water supply is a factor of its natural climatology but is further exacerbated by larger climate trends shaped by manmade influences. California has, for centuries, experienced oscillations between wet and dry periods that result in California having the greatest variations in annual precipitation of any state in the country . However, over the past century, an increase in surface temperature by 0.6-0.7° C has led to changes in California that are attributable to human GHG emissions and further affect water availability: earlier spring snow melt , an increase in percent of precipitation as rain rather than snow , warmer winter and spring temperatures , and less snow accumulation over the last fifty years .

Climate change will continue to augment the patterns of precipitation in California and intensify effects on water resources and agriculture. By early in the 21st century, the Bureau of Reclamation predicts that the Central Valley will experience a 1-degree Celsius rise in annual average temperature and a 2-degree C increase by mid-century that will likely be accompanied by a north-to-south trend of decreasing precipitation . This shift in temperature is projected to increase the frequency, intensity, and duration of droughts over the next century that will make our current water system performance levels impossible to sustain in the Central Valley . One way to prepare for the anticipated increase in drought is to study past events as an indicator of future effects. From 2012 to 2016 California experienced its worst drought in history . Water allotments were cut across the board and farmers, as the users of the majority of the state’s water, were especially hard hit . With the State Water Project and the Central Valley Project allocations cut to zero in some areas, agricultural communities in the Central Valley faced surface water reductions of an estimated 8.1 billion cubic meters a year from 2013 to 2014, amounting to a 36% reduction in surface water availability for farms . The study found that a 62% increase in groundwater extraction partially compensated for the reduction in surface water but threatened the health of California’s aquifers and moreover, still left farmers with an overall deficit of 1.9 bcm/y . This extreme event and climatological anomaly presents an opportunity to better understand how managed crops are impacted by water limitation. As lack of water will be a major limiting factor for agricultural production within the next century , patterns of crop water use and their response to reduced water availability need to be carefully analyzed so impacts to long-term food and water security can be better understood as we move into a new climate regime. Remote sensing provides new opportunities to monitor agricultural change with drought and capture spatial variations and trends in plant water use that traditional on-the ground methods like county-level reporting, lysimeters and eddy flux towers are unable to do given their limited spatial scope and significant time and labor inputs . Current crop monitoring initiatives in the United States primarily rely on imagery from earth-observing satellites such as Landsat , Moderate Resolution Imaging Spectroradiometer and the Advanced Spaceborne Thermal Emission and Reflection Radiometer to map crops and assess health and water use information . However, a new satellite, the Surface Biology and Geology Mission, has been proposed as an improvement in both spatial and spectral performance for ecosystem study . The SBG Mission will combine two sensors, a hyperspectral sensor in the visible through shortwave infrared at a 30 m resolution and a thermal sensor at a spatial resolution of 60 m for global coverage and a 5-19 day revisit. This mission has the potential to improve ability to assist crop and water managers in dynamic and diverse environments, such as the Central Valley of California, with resource accounting and drought response by capturing refined spectral information at a spatial scale that is fine enough to resolve individual fields. With the impending launch of this satellite, it is important to determine its scientific capabilities for routine observation of crops in California at a level that is of use to water managers. To test the capabilities of the SBG sensor, the Hyperspectral Infrared Imager Airborne Campaign flew the Airborne Visible/Infrared Imaging Spectrometer and MODIS/ASTER Airborne Simulator sensors on NASA’s ER-2 plane throughout California from 2013 to 2017 to simulate expected datasets from SBG . AVIRIS is a 224 band imaging spectrometer that captures spectral information from 350 to 2500 nm at ~10 nm increments .

A large public health literature suggests that exposure to PM harms health

The bio-chars used in this study contained both carbonized and non-carbonized domains, which potentially can express varied reactivities with sorbates and thus represent present different sorption mechanisms.Analysis of the sorption data suggests that monuron, diuron and linuron are likely binding to the bio-chars via multiple sorption mechanisms. The non-linearity of absorption isotherms varied between bio-chars. The non-linearity of sorption isotherms for monuron, diuron and linuron observed on the bio-chars is a characteristic of sorption processes arising from site-specific interactions occurring on the carbonized phase of the bio-char.The carbonized fraction of bio-chars is sometimes referred to as a “glassy” domain, whereas the non-carbonized soil organic matter is a rubbery domain.Generally, the sorption of organic compounds such as herbicides on carbonized phase of bio-char can be characterized by nonlinear adsorption ; however, sorption on the non-carbonized phase is better described by a partitioning mechanism that follows a linear isotherm.A lower non-linearity was observed in the low temperature bio-char sorption results and higher non-linearity was observed in the high temperature bio-char . These results indicate that a “glassy” domain sorption mechanism is involved in sorption of phenyluea herbicides to bio-char produced under high temperatures. The mechanism of low temperature produced bio-char sorption is similar to that involved in sorption to soil organic matter. The incomplete carbonization of low temperature bio-char results in bio-char with larger amounts of noncarbonized carbon than high temperature bio-char.

The microbial availability of carbon associated with the rubbery domain of low temperature bio-char is relatively higher than that associated with the carbonized phase of higher temperature bio-char.Hence,plastic garden pots the sorption capacity of phenylurea herbicides to high rubbery domain bio-char may be reduced over time due to degradation of the rubbery domain as bio-char ages after field application. Abundance of rubbery and glassy domains can also be inferred from the bio-char H/C ratios. Bio-chars with high H/C ratios, such as EB, contain larger amounts of the original organic residues. A decrease in H/C ratio indicates more complete carbonization and higher saturation in the bio-char. The 1/n value for diuron and linuron sorption data increased with the atomic H/C ratio of bio-chars, which indicates that the higher the aromaticity of sorbent, the higher the non-linearity of the sorption isotherms. It is noted that this positive correlation was observed in the higher Kow herbicides , but not in the lower Kow herbicide . This indicates that glassy domain of bio-chars plays an important role in high lipophilic herbicide sorption. The high sorption capacity of bio-chars for the phenylurea herbicides reported in this study is consistent with previously published data.Bio-char amendment to agricultural soil significantly enhanced sorption of linuron and diuron and reduced leaching of 12 kinds of phenylurea herbicides from soil to groundwater.The large capacity for bio-charsto adsorb herbicides also substantially reduced leaching of linuron, alachlor, and metalaxyl in a sandy soil. Sorption capacity of herbicides to bio-char amended soil can be lower than theoretical sorption capacity based on bio-char and soil sorption capacity measured by batch sorption experiments. Organo-mineral interactions between soil and bio-char can compete binding sites on bio-char surface with herbicides, which can diminish bio-char herbicide sorption capacity.During the ageing of bio-char, the organo-mineral interactions can also convert binding sites on bio-char surface, which can also influence herbicide sorption capacity of bio-char amended soil, both positive and negative impacts reported previously.On the other hand, bio-char amendment can reduce the effectiveness of pesticides in soil and has been shown to reduce the bio-availability of herbicides to weeds in soils.

This could require increased inputs of herbicides and increased costs of agricultural management. However, the increased adsorption capacity, if managed correctly, could possibly provide a mechanism that would permit a slow release source of herbicide from bio-char and thus lengthen the period of effectiveness of the herbicide application. Based on both lab and field scale experiments, the transport of herbicides in soil depends not only on soil properties but also climatic conditions, especially hydrological processes, such as rainfall events and soil moisture condition. These two factors can also impact the long term effects of bio-char soil amendment and interact in the ageing of bio-char. Sorption capacity of aged bio-char has been observed in some cases to decrease with time and, in other cases, remain similar to the behavior of freshly added bio-char.Based on the results above, herbicide application rates may need to be adjusted depending on how a particular bio-char ages and particular environmental conditions; this topic deserves more research. The deliberate setting of fires as a tool for agricultural management has a long history that remains ubiquitous around the world today . In modern agriculture, the principal benefit from these fires takes the form of avoided labor costs otherwise required to clear brush, remove crop residues, and manage invasive plant species . At the same time, these fires generate considerable smoke comprised of a number of pollutants that are known to be harmful to human health . Yet, the direct study of the causal relationship between agricultural fires on human health has been greatly hampered by concerns of endogeneity and the competing benefits and costs from local fires. One notable exception is the recent study by Rangel and Vogl , which examines the impacts of sugarcane harvest fires in Brazil on infant health by exploiting wind direction for empirical identification. Given the emergent literature showing that pollution can also harm a range of other human capital outcomes , the goal of this paper is to examine the impacts of agricultural fires on one important component of human capital – cognitive performance.

Our analysis of impacts on young and healthy adults in a high-stakes environment, generalizes and extends evidence from a recent working paper that examines the impact of fires on survey-based measures of cognitive decline amongst the elderly in China . More specifically, we exploit high-resolution satellite data on agricultural fires in the granary regions of China and a unique geocoded dataset on test performance on the Chinese National College Entrance Examination to investigate the impacts of fires on cognitive performance. This setting is attractive for a number of reasons. First, the majority of agricultural fires take place in the developing world where environmental controls are less stringent and the returns to human capital are generally substantial. China, in particular, is the largest grain producer in the world, with approximately one-third of all grain cropland managed through burning practices. 1 Second, the NCEE is one of the most important institutions in China. It is taken by all seniors in high school and the exam score is almost the sole determinant of admission to institutions of higher learning in China. As such, the NCEE serves as a critical channel for social mobility with important implications for earnings over the life cycle . Test takers face high-powered incentives to do as well as possible on the test and thus any impact from agricultural fires is likely to represent an impact on cognitive performance rather than effort. Finally,square pots several features of the NCEE make it particularly well suited to causal inference. The exam date is fixed, and thus self-selection on test dates are impossible. Fortuitously for our research design, the exam takes place during the height of the agricultural burning season. Moreover, students must take the exam in the county of their household registration , rendering self-selection on exam locations virtually impossible. Our NCEE data includes test scores for the universe of students who were admitted into colleges and universities between 2005–2011 from the granary regions which form the basis of our study. Despite the many virtues of our empirical setting, identifying the causal effect of agricultural fires on cognitive performance is challenging for reasons alluded to earlier. Agricultural fires are designed to reduce labor demands and improve farm profitability, both of which could also impact test performance. For example, if some agricultural labor is typically supplied by students, agricultural fires could improve test performance by providing them with more time to prepare for their exams. To address concerns of this type, we follow the approach recently pioneered by Rangel and Vogl , and leverage exogenous variation in local wind direction during the exam period. Specifically, we compare the effect of upwind and downwind fires on students’ test scores, and interpret that difference as the causal effect of pollution exposure on students’ cognitive performance net of economic impacts. The implicit assumption under this approach is that, ceteris parabus, students upwind and downwind of the fire are differentially exposed to its pollution but share equally in its economic influences. Our results suggest that a one-standard-deviation increase in the difference between upwind and downwind fires during the NCEE decreases the total exam score by 1.42 percent of a standard deviation , and further decreases the probability of getting into first-tier universities by 0.51 percent of a standard deviation. These impacts are entirely contemporaneous. Fires one to four weeks before the exam have no impact on performance. Reassuringly, neither do fires one to four weeks after the exam. The results are robust to alternative approaches for assigning pollution to test takers as well as a number of other specification checks. While a lack of pollution data from our study period does not allow us to utilize fires as an instrumental variable, data from a more recent period suggests that, consistent with evidence from Israel these cognitive impairments are likely the result of exposure to fine and coarse particulate matter. Together, these results suggest that agricultural fires impose non-trivial external costs on the citizens living near them. They also contribute to ongoing debates about the appropriate role of standardized testing in determining access to higher education and employment opportunities .

While our analysis is based on NCEE test performance, the impacts are likely much broader, touching all aspects of life that rely on sharp thinking and careful calculations. Indeed, the impacts in lower-stakes environs may well be larger as the incentives to succumb to the fatigue and lack of focus that also typically accompanies exposure to pollution are greater, and thus more likely to exacerbate any impacts on cognitive decision making. Given the importance of human capital for economic growth , these impacts should play an important role in the calculus of developing country policy makers when designing rules to manage the use of agricultural fires. The rest of the paper is organized as follows. In Section 2, we provide more background on the institutional setting. In Section 3 we describe each of the elements in our merged dataset. Section 4 describes our empirical strategy followed by our results in Section 5. Section 6 offers some concluding remarks. The practice of burning crop residues after an agricultural harvest in order to cheaply prepare the land for the next planting is commonplace across the developing world . While such burning can greatly reduce labor costs to farmers and potentially help with pest management, it also generates considerable particulate matter pollution . Particulate matter consists of airborne solid and liquid particles that can remain suspended in the air for extended periods of time and travel lengthy distances.These risks arise primarily from changes in pulmonary and cardiovascular functioning , which may, in turn, impair cognitive performance due to increased fatigue and decreased focus. Particles at the finer end of the spectrum are particularly important in our empirical setting since they are small enough to be absorbed into the bloodstream and can even become embedded deep within the brain stem . This can lead to inflammation of the central nervous system, cortical stress, and cerebrovascular damage . As such, greater exposure to fine particles is associated with lower intelligence and diminished performance over a range of cognitive domains . Consistent with this epidemiological evidence, a recent study of Israeli teenagers found that students perform worse on high-stakes exams on days with higher PM levels . As the name suggests, the NCEE is a national exam used to determine admission into higher education institutions at the undergraduate level in China. It is held annually on June 7th and 8th, and is generally taken by students in their last year of high school. In contrast to college testing in the U.S., it is almost the sole determinant for higher education admission in China.