We therefore identified Sordariomycetes as a dry farm indicator taxa, or a sort of dry farm “signature”. We included the Sordariomycetes count in models as an indication of how much the soil had shifted towards a dry farm-influenced community . AMF taxa were notably absent as discriminative taxa and PERMANOVA did not show a difference in AMF community composition between the two depths, suggesting that AMF are not limited in their dispersal down to 60cm59.After identifying Sordariomycetes as an indicator taxa for dry farming, we further explored whether multiple years of dry farming enhance soils’ dry farm signature by comparing fields that had not received external water inputs for multiple years and those which had received regular external water inputs the summer prior to the study. The extent to which Sordariomycetes were enhanced was measured by the difference between counts in dry farm and irrigated soils in the study year . We found that fields that had not received regular external water inputs the previous year showed a significantly higher difference in Sordariomycetes counts between dry farm and irrigated soils , drying rack for weed indicating that multiple years without irrigation enhance a soil’s dry farm signature.On-farm research across seven commercially managed dry farm fields allowed us to observe tomato, nutrient and soil fungal community dynamics in situ, opening a window into how dry farm systems function on working farms.
Given the long-term specialized management that farmers have tailored to their dry farm practice and fields, this on-farm approach facilitated results that reflect this management paradigm across the region and are therefore broadly applicable to dry farm management choices and outcomes on the Central Coast of California.Marketable yields per plot surprisingly did not correlate with plant spacing, which runs counter to current common wisdom in extension publications. Because spacing ranged from 15-48 inches between plants , relatively consistent yields on a per-plant basis contributed to a wide range in yields on a per-area basis . As there are very few irrigated tomatoes in the Central Coast region due to its cool, moist climate, it is difficult to compare dry farm yields to what might be found in an irrigated system in the same region. However, in 2015 , the statewide average fresh market tomato harvest was 39 T/ha, a number that is surprisingly on par with the average dry farm yield in this study . Because there is a clear tradeoff between yield and fruit quality–the highest yielding fields also had the lowest fruit quality, and increasing ammonium concentrations improve fruit quality while lowering yields–it may be difficult to increase yields above the state average while still charging consumers a premium for dry farm quality. Growers can currently charge roughly double the price per kg for dry farm-quality compared to irrigated tomatoes; therefore, short of doubling yields, current dry farmers may be reluctant to shift management to maximize yield over quality.
However, these high yields do open the possibility that dry farm management could expand to industrial-scale markets that do not rely on consumer trust in high quality produce, competing instead with irrigated production if larger scale farmers adopt dry farm practices while choosing to intentionally manage for yields over quality.Only soil nutrients at 30-60cm depth showed correlations with BER, while marketable yields and fruit percent dry weight were only influenced by nutrients below 60cm. Specifically, ammonium concentrations were associated with increased fruit quality but decreased yields and incidence of blossom end rot, while nitrate was associated with increased yields. Because soils dry down quickly in dry farm fields–available water content on average decreased by 65% in the top 30cm from transplant to midseason, while decreasing by only 16% below 60cm –plants likely devote rooting efforts to exploring deeper soils that are not too dry for efficient nutrient acquisition. Farmers also make an effort to plant transplants as deeply as possible, quickly delivering roots to depths below 30 cm. Though tomatoes root adventitiously from their stems and can therefore send out roots at shallower depths, rapidly drying surface soils likely limit nutrient uptake by adventitious roots, directing resources instead towards deeper rooting. The importance of soil nutrients at transplant at 30-60cm in predicting BER incidence, as compared to 60-100cm for yields/quality, suggests that calcium uptake occurs at an earlier stage of plant development when a higher proportion of roots were likely present at 30-60cm . Roots likely concentrated more heavily in deeper soils during fruit set and development, causing only nutrients below 60cm to show a relationship with fruit yields and PDW.
Our results also show a surprising relationship between transplant ammonium levels and fruit yields/quality. Though ammonium levels are quite low below 30cm , their negative association with yields suggests that either these low ammonium concentrations were still able to inhibit calcium/water uptake and further stress plants, as seen in studies with higher ammonium concentrations, or that higher transplant ammonium levels were indicative of other soil circumstances that negatively impacted yields. One possibility is that wetter transplant soils led to higher rates of nitrification, causing decreased ammonium levels and also higher yields due to increased water availability. While GWC was included in our models and was not significant, ammonium concentrations could in some ways be a better indicator of water availability than GWC if they more fully reflect the conditions that lead to nitrification. It is possible that, within the range of textures seen in this study, plots with higher clay content at depth inhibited plants’ ability to root deeply or led to decreased plant available water. This possibility is supported by the water x texture interaction that links plots with low clay and high GWC to increased yields. We note that the plots with the highest ammonium levels were all from one field , which exerted a strong influence on results; however, excluding Field 5 from analyses does not change the direction of nutrient coefficients, or the depth at which nutrients show a significant relationship with these outcomes. Additional research is needed to understand the unexpected relationship between ammonium concentration and harvest outcomes found here. Because nitrate levels correlate positively with yields and do not show a statistically clear relationship with BER or fruit quality, it may be tempting to conclude that farmers should increase nitrate availability in dry farm soils. However, risk of nitrate leaching must be taken into account, especially in this agricultural region that suffers from severe nitrate pollution of groundwater. Three of the seven fields in our study had nitrate levels at harvest—in just the top 15cm—above the threshold considered likely to cause groundwater contamination if that nitrate were to fully leach out of the rooting zone when it mobilizes in the first large rain event of the fall/winter wet season. These levels would likely be further accentuated by the Birch effect as soils are rewetted65. Because this first rain event typically occurs after plants are terminated, planting racks or is the terminating event itself, these systems may be particularly prone to nitrate loss when living roots are not present in the soil to recapture it. Though careful cover crop management, which is practiced by all of the farms in this study, can likely attenuate leaching, decisions to fertilize should be made with caution. Taken together, these results highlight two core challenges for dry farmers. First, there is a tension between fruit quality and yields, with conditions that lead to high yields decreasing fruit quality and vice versa. Second, it is difficult to manage soil fertility deep in the soil profile, especially when nutrients are prone to leaching.While a commercial AMF inoculant applied at tomato transplant changed AMF community composition in roots, it did not provide any benefit to yield outcomes, if anything lowering fruit quality. Diversified farm management likely made AMF communities in these soils more diverse with higher spore counts than would be seen in more industrialized systems. Altering the AMF community through inoculation may have disrupted or simply not altered functions that the endogenous community was as well or better-equipped to provide.
This result has been seen repeatedly in field research, where commercial inoculants often fail to impact agriculturally relevant outcomes, or local AMF communities outperform exogenous ones. It is also possible that, while the inoculum established enough to shift the AMF community and lower fruit quality, inocula generally will not have a large influence on dry farm tomatoes given that they are applied to surface soils while plants focus on deeper rooting, or that the specific species in the inoculant we used were not well-suited to this system. From a conceptual standpoint, there has been considerable debate in recent decades over how to best maintain agricultural productivity while also achieving systems that can maintain long-term productivity through resilience to environmental stress. These conversations often pivot around the idea of replacing industrial input-intensive agricultural practices with ecologically-based, knowledge-intensive systems. These ecologically-based systems are typically depicted as relying on on-farm biological diversity as a mechanism for increasing crops’ resilience to environmental conditions, whereas industrial systems are maintained with off-farm inputs. Even as biological diversification enters the agricultural ethos, there continues to be a pull towards achieving these biological outcomes through off-farm inputs. We typically think of chemicals and energy as the off-farm additions to conventional systems; however, products that mimic the biological effects of diversification practices can similarly be introduced from external sources rather than fostered on the farm. AMF inoculation is a prime example of how biological outcomes might be realized via external inputs. While AMF inoculation has indeed shown some benefit in more industrially managed systems, in the present study we observe that in a more diversified system, augmenting a field’s endogenous AMF community does not improve plant outcomes. Rather than replacing one external input with another , we find that farmers who already practice diversified management will likely have better luck pairing local climatic conditions with locally-adapted microbial communities.More broadly, the full fungal community in dry farm, irrigated, and non-cultivated soils were distinct, indicating different selective pressures in each soil condition. Irrigation seems to be a filter on agricultural soils, resulting in a smaller community that overlaps substantially with dry farm soils. Given that in this study only tomatoes were present in dry farm soils, while crops on irrigated soils varied from field to field, we likely overestimate the diversity of irrigated soils relative to dry farm, making this community shrinkage in irrigated soils even more pronounced. While fungal community responses to drought vary widely in the literature, there is precedent for deficit irrigation shifting bacterial communities in processing tomato fields, and natural experiments with drought conditions have led to increased fungal diversity in cotton rotations. This lower fungal diversity in irrigated systems may be driven by lower soil temperatures that are less conducive to fungal growth, or directly linked to changes in fungal competition induced by water stress that enhance diversity in dry farm systems. On the other hand, agricultural soils and non-cultivated soils seem to be distinct communities with roughly equal magnitudes of taxa numbers despite high levels of disturbance that might act as a narrowing selective pressure. Dry farm fungal diversity may be caused by external inputs that introduce non-endogenous taxa to cultivated soils. Dry farm soils were not only distinct from the other soil locations, but consistently enriched in taxa in the class Sordariomycetes. These indicator taxa formed a dry farm “signature” that was not only present in dry farm soils, but increased in magnitude in soils that had gone multiple years without external water inputs. This signature showed positive associations with fruit quality outcomes, which is of particular importance to farmers in this quality-driven system. Sordariomycetes were also associated with an increased likelihood that a plot would not have any marketable tomatoes on a given harvest day; however, as this was a rare occurrence that happened almost exclusively in the first/last weeks of harvest when yields were low for all plots, we do not expect that farmers will see an association between Sordariomycetes and yield declines. If anything, farmers may notice a slight truncation of harvest season duration in fields that have been dry farmed for several years. Sordariomycetes themselves may not be causing these outcomes, but rather point to the fact that soil microbial communities–possibly including bacteria and other microorganisms in addition to fungi–are consistently adapting to dry farm management.