Changes in biological and thermal stability of soil organic matter after long-term carbon amendments

Background/Question/Methods

Most current soil organic carbon (SOC) models assume that SOC storage efficiency (i.e. ΔSOC/ΔC inputs) is constant with C inputs. Conversely, the soil C saturation theory predicts that SOC storage efficiency, and as a result SOC stability, decreases as a soil approaches the C saturation limit. Our objective was to examine changes in the biological and thermal stability of SOC in organo-mineral complexes as soils were presumed to approach C saturation due to increasing rates of long-term C amendments.

Surface soil samples were collected from three long-term agroecosystem experiments (in Gongzhugling, Zhengzhou and Qiyang, China), where treatments have been in place since 1990: CK, no fertilizer; NPK, mineral NPK fertilizer; NPKM, mineral NPK fertilizer and manure; 1.5NPKM, 1.5× (NPK fertilizer and manure). These treatments generated increasing amounts of C inputs to soil, either directly from manure additions or indirectly through increased plant productivity from fertilization. The silt+clay fraction (< 53 µm) was isolated by wet-sieving, and subjected to a short-term laboratory incubation and thermal analysis to assess relative SOC stability. During the incubation, respired CO₂ was measured at incubation days 1, 4, 7, 10, 15, 20, 30, and 40, and used to determine potentially mineralizable C. In the thermal analysis, simultaneous thermogravimetry (TG), differential scanning calorimetry (DSC), and CO₂ release during combustion were used to determine several thermal stability indices (relative mass loss; TG-T₅₀, temperature at which half of the exothermic mass loses; net exothermic energy content; DSC-T₅₀: temperature at which half of the exothermic energy is released; CO₂-T₅₀, temperature at which half of CO₂ released during combustion).

Results/Conclusions

With increasing rates of C inputs, potentially mineralizable C increased by 70-111% in Gongzhuling, by 3-44% in Zhengzhou, and by 25-131% in Qiyang, compared to the CK treatment. However, when normalized to initial soil C, potentially mineralizbale C increased only by 19-67% in Gongzhuling, by 5-17% in Qiyang, with no significant increase in Zhengzhou. Results of the thermal analyses were generally consistent with the biological indices of SOC stability. TG-T₅₀ decreased and net exothermic energy content increased with increasing C inputs in all three sites. DSC-T₅₀ and CO₂-T₅₀ trends differed among sites. The results show that with increasing rates of C inputs, changes in organo-mineral SOC stability are site-specific in these three sites and are likely controlled by differences in mineralogy.