Biochar and nitrogen application rates effect on phosphorus removal from a mixed grass sward irrigated with reclaimed wastewater.

This 2-year (2017 and 2018) field study evaluated biochar and nitrogen application rates effect on herbage phosphorus (P) and nitrogen (N) removal from a mixed-grass sward of tall fescue [Schedonorus arundinaceus (Schreb.) Dumort] and Kentucky bluegrass (Poa pratensis L.) irrigated with treated wastewater. Treatments used in this study carried out at the Main Station Field Laboratory, Reno, NV were three biochar application rates (0, 8.9, and 17.8 Mg/ha), and three N rates (0, 80, and 120 kg N/ha) arranged in a 3 × 3 factorial in a randomized complete block design experiment with four replications of each treatment combination. Responses were considered different P < 0.05. There was a linear increased in soil volumetric water content as biochar rate increased from 0 to 17.9 Mg/ha. However, biochar application rate did not affect the quantity of biomass produced, forage tissue P and N concentrations, P and N removal or interact with the other experimental variables of N rate and year to influence the response variables. There was, however, an N rate effect (P < 0.05) on biomass production and it was greater for the 80 and 120 kg N rate (average = 8.3 Mg DM/ha) relative to the 0 kg N/ha rate (6.0 Mg DM/ha). Further, cumulative P removal for the 80 and 120 kg N rate (average = 48.9 kg/ha) was greater than the 0 kg N/ha rate (38.1 kg/ha), and cumulative N removal was in the order 120 kg N/ha (321.1 kg/ha) > 80 kg N/ha (267.4 kg/ha) > 0 kg N/ha (187.8 kg/ha). There was a trend for a biochar × N rate interaction on soil P concentration and it tended to be greater for the combinations 8.9 and 17.8 Mg/ha biochar rates and 80 and 120 kg N/ha rates compared to the unamended control. Even though our study did not reveal a definitive effect of biochar on the major response parameters (biomass, tissue P and N concentrations) evaluated, the trend for a biochar × N rate interaction on soil P concentration offers hope that biochar-amended soils coupled with appropriate N fertilization will be effective in P retention on agricultural landscapes irrigated with treated wastewater.