Microbial residue indices down the soil profile after long-term addition of farmyard manure and mineral fertilizer to a sandy soil

Long-term organic fertilization may control the accumulation of organic matter in subsoil. The objective of this study was to evaluate the effects of long-term farmyard manure application in comparison with mineral fertilization on the accumulation of amino sugars as indices for microbial residues down to 1 m depth at a sandy site that exhibits highly heterogeneous pH conditions. In relation to maximum values in topsoil at 90-100 cm depth, the SOC content decreased to roughly 24% and the total N content to 16% of the maximum values, leading to an increased soil C/N ratio from 11 to values around 16 in all treatments. The relative contribution of microbial residue C to SOC decreased with depth from 68% at 0-25 cm to 24% at 50-100 cm. In the subsoil, the stocks of microbial residue C were increased by manure in comparison with mineral fertilization, but not the stocks of SOC. This suggests that manure-induced priming effects increase the microbial turnover at 50-100 cm depth. Manure fertilization promoted the formation of bacterial residues in the topsoil at 0-25 cm depth, but not in the subsoil. Below the topsoil, the fungal C to bacterial C ratio decreased from 2.6 at 0-25 cm depth to 2.1 at 50-100 cm depth. Below the topsoil, the ratio of fungal to bacterial residues continuously decreased with depth from 2.7 to 1.7 at 90-100 cm depth, without fertilizer effects. Possible reasons for this decrease, such as effects of pH on the subsoil microbial community, a higher sensitivity of fungi to the absence of fresh organic matter or to an unfavourable composition of the subsoil atmosphere, need further investigations.