Isotope-Based Insights into Old Questions about Transport and Transformation of Plant Inputs to Soil Based on Isotope Studies.

As recognition grows of the importance of subsurface soils to carbon cycling, so too does interest in the source of organic matter in deeper horizons. The source of OM to deeper soils has major implications for OM stocks and residence times. Moreover, understanding the contributions of roots and leaves to SOM is integral to understanding how changing plant allocation will affect soil carbon cycling. If leaf litter dominates, slow transport could explain old ages of deep soil OC. Nevertheless, the historical focus of experiments, models, and theory has been on aboveground inputs at the surface and not on connections to deeper soil horizons. Isotopes, for example in ¹³C-labeled litter or the ¹⁴C value of SOC pools or microbial respiration, offer a powerful tool for tracing the fate of plant inputs in soil. Here, we synthesize data from lab and field studies in forest and grassland soils where we used ¹³C, ¹⁴C, and ¹⁵N to quantify how litter C and N are transported and incorporated into SOM in deeper soil horizons. In a conifer forest soil, after 10 years, only 3% of litter C applied to the O horizon was recovered in the A horizon. Looking deeper, after 2.5 years, no litter C was detected below the depths of insertion (15, 55, and 95 cm). In grassland lysimeters, <0.15% of litter C was recovered in leachate. In the lab, a year’s worth of rainfall leached only 5% of surface litter C to the top 8 cm of soil, most of that in the top 2 cm. These studies [and others] find that the majority of aboveground litter C does not make it to deeper soil horizons. ¹⁴C-studies in five deciduous forests confirmed that in situ inputs such as roots are the main source of soil carbon--and should be treated as such in models. Since root litter typically decomposes slower than leaf litter, and arrives in deeper soils as larger macromolecules, more root litter input might mean less efficient microbial processing, which affects mechanisms of long term carbon storage. However, the contributions of aboveground litter N are a different story. In the 10 year experiment, 8% of litter N applied to the O horizon was retained in the A, and in the lab leaching experiment, 30% of litter N was recovered in the top 10 cm of soil. This is consistent with widely observed decreases in SOM C:N ratios with depth.