Impact of pine chip biochar on trace greenhouse gas emissions and soil nutrient dynamics in an annual ryegrass system in California

Manure generated by dairy cattle is a useful soil amendment but contributes to greenhouse gas (GHG) emissions and water pollution from nutrient leaching. In order to assess the impact of pine chip biochar produced at a peak temperature of 550 °C when added to a dairy grassland system, a one-year field study was conducted on a sandy loam soil under annual ryegrass (Lolium multiflorum Lam.) grown for silage in Petaluma, California. Manure was applied to all plots at a rate of ca. 150 m3 ha−1 (410 kg N ha−1). Control plots received no biochar, high application biochar plots (HB) received biochar (with a 17% ash content) at a rate of 18.8 t ha−1, and low application biochar plots (LB) received the same biochar at 5.7 t ha−1. Although the HB plots demonstrated the lowest cumulative nitrous oxide (N2O) and methane (CH4) emissions, there was no significant difference between treatments (p = 0.152 and p = 0.496, respectively). Soil pH results from samples collected throughout the year indicated a significant treatment effect (p = 0.046), though Tukey test results indicated that there was no difference between mean values. Soil total carbon was significantly higher in HB plots at the end of the experiment (p = 0.025) and nitrate (NO3−) intensity throughout the year (which expresses potential exposure of NO3− to the soil microbial community) was significantly lower in HB plots compared to the control (p = 0.001). Annual cumulative potassium (K+) loss from HB plots was significantly higher than from the other treatments (p = 0.018). HB plots also demonstrated a short-term increase in phosphorus (P) and ammonium (NH4+) in leachate during the first rainfall event following manure and biochar application (p < 0.0001 and p = 0.0002, respectively) as well as a short-term decrease of NO3− in leachate during a heavy rainfall event following a long dry spell (p = 0.036), though differences between treatments for cumulative nutrient losses were not significant (p = 0.210, p = 0.061, and p = 0.295, respectively for P, NH4+, and NO3−). These data indicate that biochar produced from pine wood chips at 550 °C having high ash content (17%) is not likely to impact GHG emissions in systems with high manure application rates. Further research should be conducted in order to investigate the impact of biochar amendment on the dynamics and mobility of nutrients applied in subsequent repeated applications of dairy manure.