Increasing soil organic matter content in mine soil through pig manure addition

Abstract

Mine soils in southeast Spain have scarce vegetation due to very poor properties such as extremely low soil organic matter (SOM) (< 0.6 g carbon kg-1 soil), low pH, high salinity and metal contents. Also in southeast Spain, there is an economically-important pig husbandry faced with challenges to manage the large volumes of industry-generated animal wastes. This study will present the results of a leaching experiment to assess the retention and release of nitrogen and carbon from pig manure added to undisturbed column of mine soil. We excavated three columns (15-cm diameter and 30-cm length) from a representative mined site. The columns were amended with single (7 % by mass) and double doses of pig manure, and leached weekly with distilled water for 10 weeks to simulate annual rainfall events in the study area. Leachates were collected and analyzed for pH, electrical conductivity, redox potential, and contents of selected anions and metals. However, we will limit this presentation to carbon and nitrogen to quantify the potential contribution of pig manure addition to the build up soil organic matter in mine soils. Results showed that after addition of pig manure in the soil surface, soil pH increased from 2.2 to 4.0 after 11 weeks (single dose) and to 5.2 at week 21 (double doses). Significant increased were observed in total nitrogen contents in both single and double doses, 1.14 g kg-1 (900 %) and 1.40 g kg-1 (1100 %), respectively. Total carbon contents increased to 18.6 g kg-1 (3200%) in single dose and to 16.4 g kg-1 (2800%) in double doses. Nitrogen and carbon in soils had weekly rate of increases of 0.1 and 2.0 g kg-1, respectively. Moreover, C/N ratio increased from 5 to 12 at the end of the experiment. Leachates had significantly higher weekly release of NO3- than total dissolved organic carbon (DOC) during the first 6 weeks of leaching. Weekly rate of releases (mg L-1) were 127 (NO3-) and 5.2 (DOC) in single dose, and 35 (NO3-) and 2.8 (DOC) for double doses. Leachates contained NO3- less than the 50 mg L-1 threshold established by FAO. These results suggest that addition of pig manure may significantly accelerate the build up of SOM in mine soils without endangering the release of NO3- into sub-soil or groundwater in semiarid regions. Once there is sufficient SOM, mine soils will have an environment hospitable to various ecosystems including plant colonization and microbial community needed for its physical stability. Pig manure amendment of mine soils can be an ecologically-sound means of managing the large volume of wastes generated by the pig industry in southeast Spain.