Effects of whole-orchard recycling on nitrate leaching potential in almond production systems.

Inefficient nitrogen (N) fertilization and irrigation have led to unhealthy nitrate levels in groundwater bodies of agricultural areas in California. Simultaneously, high commodity prices and drought have encouraged perennial crop growers to turnover less-productive orchards, providing opportunities to recycle tree biomass in situ and to use high-carbon (C) residues to conserve soil and water resources. Although climate change adaptation and mitigation benefits of high-C soil amendments have been shown, uncertainties remain regarding the benefits and trade-offs of this practice for N cycling and retention. We used established almond [Prunus dulcis (Mill.) D. A. Webb] orchard trials on Hanford fine sandy loam with short-term and long-term biomass recycling legacies to better understand the changes in N dynamics and retention capacity associated with this practice. In a soil column experiment, labeled N fertilizer was added and traced into various N pools, including microbial biomass and inorganic fractions in soil and leachate. Shifts in microbial communities were characterized using the abundance of key N cycling functional genes regulating nitrification and denitrification processes. Our findings showed that, in the short term, biomass recycling led to N immobilization within the orchard biomass incorporation depth zone (0-15 cm) without impacts on N leaching potential. However, this practice drastically reduced nitrate leaching potential by 52%, 10 yr after biomass incorporation without an increase in N immobilization. Although the timing of these potential benefits as a function of microbial population and C and N biogeochemical cycles still needs to be clarified, our results highlight the potential of this practice to meaningfully mitigate nitrate discharges into groundwater while conserving soil resources.

Orchard recycling improves climate change adaptation and mitigation potential of almond production systems.

There is an urgent need to develop climate smart agroecosystems capable of mitigating climate change and adapting to its effects. In California, high commodity prices and increased frequency of drought have encouraged orchard turnover, providing an opportunity to recycle tree biomass in situ prior to replanting an orchard. Whole orchard recycling (WOR) has potential as a carbon (C) negative cultural practice to build soil C storage, soil health, and orchard productivity. We tested the potential of this practice for long term C sequestration and hypothesized that associated co-benefits to soil health will enhance sustainability and resiliency of almond orchards to water-deficit conditions. We measured soil health metrics and productivity of an almond orchard following grinding and incorporation of woody biomass vs. burning of old orchard biomass 9 years after implementation. We also conducted a deficit irrigation trial with control and deficit irrigation (-20%) treatments to quantify shifts in tree water status and resilience. Biomass recycling led to higher yields and substantial improvement in soil functioning, including nutrient content, aggregation, porosity, and water retention. This practice also sequestered significantly higher levels of C in the topsoil (+5 t ha-1) compared to burning. We measured a 20% increase in irrigation water use efficiency and improved soil and tree water status under stress, suggesting that in situ biomass recycling can be considered as a climate smart practice in California irrigated almond systems.

Varying Influence of Dairy Manure Injection on Phosphorus Loss in Runoff over Four Years.

Surface application of manure on no-till farms can exacerbate P losses in runoff, contributing to the eutrophication of surface waters. We monitored 12 400-m field plots over 4 yr to compare P losses in surface runoff and lateral subsurface flow with shallow disk injection and broadcast application of dairy manure. Given the substantial variability in annual P losses, as well as a gradual, annual buildup of residual soil test P, significant differences in runoff P losses were detected in only 1 of 4 yr: in 2014, total P losses in runoff were 68% greater from broadcast manure plots than injected manure plots. Dissolved and particulate P were roughly even in their contribution to runoff. Even so, there were significant relationships between annual dissolved P losses and P in the soil surface, which pointed to soils as a regular source of P in runoff. Overall, results confirm the potential for injection to reduce P loss in runoff relative to broadcast application, but because in a few sampling dates injection resulted in greater losses, this study also highlights the importance of assessing mitigation benefits of manure application practices over longer timeframes.

Accumulation of silymarin in milk thistle seeds under drought stress.

MAIN CONCLUSION: According to the results obtained in this study, drought stress can enhance the accumulation of silymarin in milk thistle seeds. Moreover, under drought stress, the share of silybin increased which possess the greatest degree of biological activity among the silymarin components. Silymarin, an isomeric mixture of flavonolignans found in milk thistle (Silybum marianum (L.) Gaertn) seeds, has been used for its hepatoprotective effects for more than 2,000 years. Biosynthesis and accumulation of active substances like silymarin in plant tissues highly interacts with the environmental conditions. Effects of moderate and severe drought stress (based on soil moisture depletion) on silymarin content and composition in milk thistle seeds were evaluated in a field study. Averaged across treatments, milk thistle seeds contained 19.3 g kg(-1) silymarin. Drought stress enhanced silymarin accumulation in milk thistle seeds. Plants grown under moderate and severe drought stress treatments contained 4 and 17 % greater silymarin than those grown in well-watered condition, respectively. Greater content of sylimarin in stressed plants was attributed to more contents of silybin, isosilybin and silychristin, while silydianin content was lower under drought condition. According to the results obtained in this study, drought stress enhanced accumulation of silymarin in milk thistle seeds and improved its quality by increasing the share of silybin, which possess the greatest degree of biological activity among the silymarin components.