Changes in quality and quantity of soil organic matter stocks resulting from wastewater irrigation in formerly forested land

Publication date: 15 November 2017
Source:Geoderma, Volume 306
Author(s): Arturo Sánchez–González, María Chapela–Lara, Edgardo Germán–Venegas, Ruth Fuentes-García, Federico del Río-Portilla, Christina Siebe
Land use changes often diminish soil carbon stocks and alter soil organic matter (SOM) quality affecting soil productivity. Therefore, intensively managed agricultural systems are intended to maintain and even increase SOM stocks. Here we compared soil organic carbon (SOC) stocks and their quality in fields under natural vegetation (mesquite forest and xerophytic shrubs), converted first to rain fed maize cultivation and then to irrigated alfalfa-maize rotation systems in a semiarid region of Central Mexico. Fields under the alfalfa-maize rotation have been irrigated for different lengths of time (15, 35, 51, 85 and 100years) with untreated wastewater from Mexico City. We tested if the SOC stocks of the irrigated land use system were similar or even larger than those found under natural vegetation, and investigated if the quality of the SOM changed under the different covers. We collected composite soil surface samples and soil profile samples from fields under the different land uses and irrigation lengths and determined their organic carbon concentrations; we also quantified root residues and analyzed their bromatological composition. Mid infrared spectra of the soil samples were recorded and analyzed by multivariate procedures to determine changes in SOM quality. We found the smallest SOC stocks in soils under rain fed maize (mean: 45Mgha ⁻¹ in 30cm depth). In the alfalfa-maize rotation SOC stocks increased 1.5 fold during the first 40years under wastewater irrigation, thereafter reaching an apparent new steady state, with similar C stocks to soils under xerophytic shrub cover (65Mgha ⁻¹). SOM in rain fed soils was more hydrophobic and more metabolized by microorganisms, while the SOM in irrigated soils had a more hydrophilic character. Lignin in surface horizons of long term irrigated soils has a lesser degree of condensation than in deeper soil horizons or in soils irrigated for shorter periods of time, indicating a less intensive microbial degradation. We conclude that wastewater irrigation increases SOM stocks due to the greater lignin rich root biomass produced by alfalfa in this land use system, which helps to recover the SOC loss of 17–34% of the rain fed agriculture system.