Long-term effects of liquid swine manure on soil organic carbon and Cu/Zn levels in soil and maize

Swine feed is often supplemented with Cu and Zn, but > 90% of the ingested metal is excreted, ultimately accumulating in soil treated with liquid swine manure (LSM) to meet nitrogen (N) requirements. A maize field experiment was conducted between 2002 and 2015 in the Ebro Valley (north-east Spain) to determine the effects of long-term LSM application at two rates [30 and 50 m(3) ha(-1), equivalent to similar to 220 (218) and similar to 390 (389) kg N ha(-1)] compared to 0 and 300 kg N ha(-1)mineral fertilizer. The highest Cu concentration in plants (5.03 mg kg(-1)) was observed following the 300 kg N ha(-1) treatment. The treatments did not affect plant Zn levels (similar to 21 mg kg(-1)). All whole-plant and grain metal concentrations were lower than threshold values for animal and human ingestion (30 mg Cu kg(-1)and 500-1300 mg Zn kg(-1)). After 12 years, the soil organic carbon (SOC) of 13 g kg(-1) in the first year (2002) increased by 19% and 26% at the low and high LSM rates, respectively. Mineral fertilization (300 kg N ha(-1)) achieved a 22% increase in SOC and also the 0 kg N ha(-1)control treatment caused the SOC to rise by 11%. LSM applications do not seem to have adverse effects on maize yields after 12 years of applications.