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Sustainable agriculture, characterized by farming profitably while minimizing damage to the environment, is not easy to practice. “Conventional agriculture” worldwide is commonly considered to involve numerous practices that have potential to damage the environment. “Sustainable agriculture” attempts to find a range of conservation agricultural practices which may mitigate some of the adverse impacts of land use intensification. Among the wide range of the conservation agricultural practices, biochar may prove a key and accessible input for sustainable agriculture, as it could efficiently sequester large amount of carbon in soil over the long-run, thus improving soil fertility, crop productivity, and mitigate global warming. In addition, biochar may enhance crop productive capacity of soils under different biotic and abiotic stresses, and advance global food security. The main focus of this report is providing a critical scientific review of the current state of knowledge regarding the effects of biochar application to soils on soil properties, processes and functions. Wider issues, including atmospheric emissions and occupational health and safety associated to biochar production and handling, are put into context. The aim of this review is to provide a sound scientific basis for policy development, to identify gaps in current knowledge, and to recommend further research relating to biochar application to soils.

Partial rootzone drying (PRD) has been reported to potentially improve crop water use efficiency (WUEcrop) compared to full irrigation in different fruit trees; however, field studies on the effect of PRD in citrus are scarce. In this field study, three irrigation strategies were tested in an orchard of mature grapefruit trees during two consecutive seasons (2013/2014 and 2014/2015): drip PRD (two drip lines, alternating irrigation between lines every month), microsprinkler irrigation, and double-line drip irrigation (control). Irrigation was applied during the fruit enlargement stage (April–August). The aims of this field study were: 1) to provide a quantitative comparison of irrigation water productivity among irrigation treatments; and 2) to study their effects on fruit quality, yield, tree growth, and flowering. Drip PRD saved 43 to 47% of irrigation water compared to control irrigation, and microsprinkler irrigation saved 12 to 18% of water compared to control irrigation PRD-irrigated trees maintained or increased yield compared to microsprinkler-irrigated and control trees, depending on the experimental season. Therefore, WUEcrop in PRD-irrigated trees was significantly higher than in control and microsprinkler-irrigated trees at the end of both seasons. Fruit and juice quality parameters were statistically similar among all treatments. PRD irrigation did not reduce the flowering potential of the trees although it delayed the onset of flowering in trees relative to the other treatments in 2013/2014. There was a similar timing of flowering among treatments in 2014/2015 and there were no differences in vegetative growth among the irrigation treatments by the end of March (86 Julian data) in both experimental seasons. Our results suggest that PRD can be economically beneficial for citrus growers who use double-line drip irrigation systems, and a strategy to sustain tree growth, tree health and yield during seasons of extreme drought or when high water restrictions are placed in citrus-producing areas.

Subtropical and tropical climates are becoming more vulnerable to drought and salinity problems. Of the crops produced in these climates, citrus is especially sensitive to increasing salinity. In times of water scarcity and periodic drought alternative water sources that may be lower in quality may be required for irrigation, leading to reduced yield unless a mechanism of increasing salinity tolerance is found. The salinity tolerance of citrus rootstocks differs depending on species and cultivar in their ability to exclude or sequester toxic ions. Identifying rootstocks that are more salt tolerant is important to citrus production in susceptible areas. ‘Olinda’ Valencia trees budded to three rootstocks (sour orange [SO], C22, and C146) and the ungrafted rootstocks were subjected to 6 months of saline irrigation. Simulated brackish water at 1, 3, 5, and 10 dS m −1 along with a 0 dS m −1 control were used to irrigate trees over the course of the study. Throughout this time period chlorophyll fluorescence, stomatal conductance, electrolyte leakage, and SPAD were measured to identify how trees responded to different levels of salinity stress. Chlorophyll fluorescence, stomatal control, and SPAD decreased with increased salinity, while electrolyte leakage increased. These responses were more pronounced in grafted trees than ungrafted trees. C22 and C146 rootstocks showed more tolerance to increases in salinity than the SO rootstocks. The ‘Olinda’ Valencia scion reduces rootstock tolerance to salinity, however, trees grafted to the C22 and C146 rootstocks showed more salinity tolerance than those grafted to SO rootstocks. This study emphasizes that both rootstock and scion should be considered when selecting trees for saline conditions.

Grove floor management practices play a significant role in citrus physiology, root distribution, and overall tree health. Among the various agricultural practices evaluated in this study, black plastic mesh groundcovers showed increasing soil moisture availability and temperature, as well as improving plant growth and yields. Raised bed practices have also been used to increase soil drainage and porosity to promote root proliferation in compacted soils. While these practices have been primarily studied on annual or row crops, their introduction into grove systems has been slow, even the positive results obtained with these practices. Citrus are managed very differently from row crops, with long-term practices and management techniques often negatively impacting root growth. Citrus management practices using raised beds and black plastic mesh groundcovers have shown great promise in improving citrus production and tree growth. However, few studies have investigated the impact of these practices on root growth and morphology and how these practices affect aboveground growth and production. In this experiment, we aimed to evaluate how different practices affected root growth and how this, in turn, impacted aboveground parameters. To accomplish this, we examined root distribution and in-growth (root growth inside a hand-made core) in four different citrus grove floor management systems; raised bed with groundcover, raised bed without groundcover, flatbed with groundcover, and flatbed without groundcover at various times throughout the growing season for three years. We then assessed how each treatment parameter affected root characteristics and how this affected the aboveground and soil measurements. Root analysis with WinRhizo® software showed significant differences between management systems. Raised beds with black plastic groundcovers increased root growth and density throughout the study. Seasonal in-growth core data indicated that the period from Fall-Spring showed significantly higher root growth than the Spring-Fall period, indicating seasonal growth and senescence of roots. The Spring-Fall cores were also less influenced by management treatment than the other sampling dates. While short-term growth varied with season, the effects of raised beds with plastic groundcovers positively influenced tree growth, root growth, and morphology over the long-term.

Salinity and drought are among the greatest threats hindering global food security. Over the last decade, biochar use in agriculture has received increasing attention and enhancing biochar through composting and chemical modification offers a potential strategy for improving soil productivity. However, there is limited knowledge on how residual sulfur-enhanced biochars alone, or in combination with effective microorganisms (EMs) impact on soil properties, and how this influences agricultural productivity in the short and long-term. To investigate this, a field experiment was conducted in 2017 to study the effect of residual sulfur-enhanced biochar (RSBC). A 5:100 (w/w) mixture of elemental sulphur (S) and citrus wood biochar (BCH), respectively combined with EMs was applied to soil. The effects on soil properties, growth, and productivity of salt-stressed pepper plants was then evaluated to determine if supplementation with RSBC and/or EMs bio-protected plants could mitigate the deleterious effects of soil salinity. These applications resulted in several beneficial effects which included: improving dehydration tolerance, nutritional status, and photosynthetic efficiency, while significantly reducing Na+ and Cd2+ concentrations in plants. The integrated RSBC + EMs treatment significantly increased plant growth, and productivity, macro- and micro-nutrient concentration, as well as, dehydration tolerance and irrigation use efficiency. A reduction in Na+ and Cd2+ uptake was also observed in conjunction with a concomitant increase in N, P, K+, Ca2+, Fe, Mn, Cu and Zn uptake. Overall, applications of residual sulfur-enhanced biochar combined with effective microorganisms show short-term beneficial effects on peppers can be attained in the presence of high salinity. This study opens up a wide avenue for research exploration; subsequent studies to examine the benefits of long term or repeated applications and could yield even more positive applications or results.

As population and agricultural pressure on high quality fresh water resources increases, it becomes more important to identify rootstocks and production techniques that allow the usage of lower quality water in irrigated agriculture. Water of 0, 1, 3, 5 and 10 dS m −1 salinity was applied to grafted (with Olinda Valencia ( Citrus sinensis var. Olinda) scion) and ungrafted sour orange ( Citrus aurantium L.), C22, and C146 ( Citrus sunki Hort. Ex Tan. × Poncirus trifoliata L. Raf. ‘Swingle’) trees to evaluate the effect of grafting and rootstock variety on salinity tolerance. Grafted rootstocks had a lower survival rate at higher salinity concentration than ungrafted trees and effects were dependent upon tree variety. Height growth rate and standing leaf area of grafted citrus trees declined with increased salinity concentrations while the response of ungrafted and grafted citrus trees was variety dependent. Mortality increased and standing leaf area decreased with increasing tissue Na + and Cl − concentrations. Mortality in response to salinity was greater in grafted rootstocks than mortality of ungrafted trees. Overall, C22 and C146 rootstocks and ungrafted trees were found to be more tolerant of saline conditions than grafted SO rootstock or ungrafted SO trees.

Improving sustainable citrus production through soil and water conservation strategies such as plastic mesh groundcovers and raised planting beds has not been thoroughly evaluated. To determine the impacts of these practices on tree growth parameters, fruit yield, and soil environmental conditions we monitored a grapefruit grove (Citrus paradisi Macf. cv.‘Rio Red’) for five years after planting. Trees were planted under four grove floor management strategies that included flat beds with no plastic mesh groundcover (traditional practice), flat beds with groundcover, raised beds with no groundcover, and raised beds with groundcover. The use of ground cover increased soil temperature and moisture content and led to higher tree growth rates and yields relative to uncovered grove floors. Raising the planting bed to approximately 45 cm higher than the surrounding field reduced soil deposition from irrigation and improved soil properties, but caused fluctuations in soil moisture due to improved water infiltration of heavy clay soils compared to the flat beds. Despite the positive effects of the use of ground cover on yields, there was a trend of alternate bearing, smaller and sweeter fruit sizes for trees under this planting design. The traditional method of planting (flat bed with no groundcover) resulted in smaller trees as measured by tree height, canopy circumference, and trunk diameter. While yields were moderate, they were fairly consistent across years and increased slightly each year of the study. These results show that adoption of plastic mesh groundcovers in citrus or other tree fruit production could potentially improve water conservation while increasing production.