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 .