Many field studies exploring biochars' effects on plant productivity and soil quality have been limited to just one or two seasons, particularly in temperate agroecosystems, and have not shown how such impacts change as biochars age in the soil. Therefore, we investigated the lasting effects of a walnut shell (WS) biochar on crop yields and soil nutrient cycling and availability over four years in a field experiment. Long-term plots of a tomato-corn rotation were established in a 2x2 factorial design of treatments i) with or without WS biochar amendment and ii) fertilized with mineral fertilizer (MF) or composted poultry manure (CP). Biochar was applied once in 2012 (Year 1) at a rate of 10 t ha −1. Crop yields were measured over four seasons, and soil samples were analyzed for ammonium (NH 4 +-N) and nitrate (NO 3 −-N) concentrations and for other nutrient parameters, including exchangeable K + , Ca 2+ , PO 4-P, SO 4-S, each year. WS Walnut shell biochar had an effect only in Year 2 2013, one year after biochar application, when it increased corn yields by ~8% in both MF and CP fertilizer systems and increased

A B S T R A C TPeat moss has historically been a key component of soil-free substrates in the greenhouse and nursery industries. However, the increasing expense of peat, negative impacts of peat mining on wetland ecosystems, and growing perception of peat as unsustainable have led to investigation for alternatives. Biochar (BC) is a promising substitute for peat, yet the majority of studies examine additions of BC to peat-based substrates rather than replacing the peat component or employ relatively low substitution rates. Furthermore, at high substitution rates the alkalinity common to many BCs may increase substrate pH and adversely impact plant production. We evaluated BC substitution for peat and pH adjustment of resulting substrates on marigold (Tagetes erecta L.) performance under standard greenhouse conditions. A high pH (10.9) softwood BC (800°C) was substituted for peat in a standard 70:30 (v/v) peat:perlite mixture at 10% total volume increments. Substrate pH was either not adjusted or adjusted to pH 5.8 using a BC by-product, pyroligneous acid (PLA). Germination was inhibited in pH adjusted substrates with high BC substitution (50-70% total substrate volume) likely due to higher dosages of PLA needed to neutralize pH. At harvest (flowering stage, 9 weeks) the initial pH gradient (4.4-10.4) in substrates that were not pH adjusted had converged to pH 5.6-7.5, and BC substitution for peat did not negatively impact marigold biomass or flowering. At low substitution rates (10-30% total substrate volume), marigold biomass and leaf SPAD values were greater than the control peat-perlite mixture (0% BC). This study demonstrates that softwood BC can be considered as a full replacement for peat in soil-free substrates, and even at high rates (70% total substrate volume) does not require pH adjustment for marigold production. Crop-and BC-specific considerations and economic potential should be investigated for wider application.

Biochar amendments to soil have been promoted as a low cost carbon (C) sequestration strategy as well as a way to increase nutrient retention and remediate contaminants. If biochar is to become part of a long-term management strategy, it is important to consider its positive and negative impacts, and their trade-offs, on soil organic matter (SOM) and soluble C under different hydrological conditions such as prolonged drought or frequent wet-dry cycles. A 52-week incubation experiment measuring the influence of biochar on soil water soluble C under different soil moisture conditions (wet, dry, or wet-dry cycles) indicated that, in general, dry and wet-dry cycles increased water soluble C, and biochar addition further increased release of water soluble C from native SOM. Biochar amendment appeared to increase transformation of native SOM to water soluble C, based on specific ultraviolet absorption (SUVA) and C stable isotope composition; however, the increased amount of water soluble C from native SOM is less than 1% of total biochar C. The impacts of biochar on water soluble C need to be carefully considered when applying biochar to agricultural soil.

Soilborne pathogens can devastate crops, causing economic losses for farmers due to reduced yields and expensive management practices. Fumigants and fungicides have harmful impacts on the surrounding environment and can be toxic to humans. Therefore, alternative methods of disease management are important. The disease suppressive abilities of composts have been recognized for several decades, and significant research has been done in order to identify substrates with effective suppression. The mechanisms of suppression are mainly biological, but abiotic aspects of the composts, such as pH, carbon to nitrogen ratio, and maturity, interact with pathogenic and biological control processes and determine efficacy of suppression. For example, Fusarium wilt is aggravated by high ammonium‐N composts (Cotxarrera et al., 2002), and mature composts with low levels of labile compounds more effectively suppress Rhizoctonia damping‐off (Trillas et al., 2006). Identification of these abiotic factors can increase efficacy of disease suppression of composts. In addition, inoculating composts with biological control agents, such as Trichoderma, has been found to increase suppressive ability in many cases.