Soil textural control on moisture distribution at the microscale and its effect on added particulate organic matter mineralization

Abstract

Soil texture determines the location of moisture in the soil matrix. At lower moisture content, the contact between soil water and particulate organic matter (POM) might be critical for POM mineralization, because such areas of contact could function as a ‘bridge’ for transport of moisture, substrates, enzymes and microorganisms. Additionally, water absorption by porous POM itself likely also depends on soil texture and this may thus bring about another indirect textural control on POM mineralization. However, the relevance of such indirect soil textural controls on POM mineralization has not been investigated. Here we compared mineralization of 13C-labeled maize-straw particles (Cmaize-min) in a loamy sand and silt loam soil at three levels of water-filled pore space (20%, 40% and 60%WFPS). The distribution of discrete maize-straw particles and moist soil volume was assessed at the microscale using X-ray μCT and contrast-enhancing agents to visualize soil moisture. We hypothesized that a lower contact between POM and moist soil minerals in the loamy sand soil would lead to a reduced POM mineralization compared with the silt loam soil. On the other hand, we expected that POM itself could attract more moisture from the surrounding loamy sand soil due to a higher water potential than in the silt loam soil at equal WFPS level. Contrary to our expectation, there was less contact between zones of moist soil and the added POM in the silt loam soil. But as expected, POM was able to attract more water from the loamy sand soil, and it preserved a moist state even when soil was at a low moisture content, which may explain the surprisingly limited inhibition of Cmaize-min in drier loamy sand soil. This finding suggests that such a ‘sponge-effect’ of POM may be a relevant yet largely overlooked element in studying the relationship between soil moisture and POM mineralization. Contrary to our expectation, Cmaize-min was higher in the silt loam soil texture across all three moisture levels (P < 0.05). At low moisture content particularly, fungal and actinobacteria biomarker abundances were elevated in the silt loam soil. A shift towards a fungi-oriented microbial community could explain the higher Cmaize-min rates in the silt loam soil under dry than moist conditions. Overall, our results showed that mineralization of added POM was surprisingly independent of soil moisture, but explanatory pathways were probably very much soil texture dependent.