Decomposition of carbon-14-labeled organic amendments and humic acids in a long-term field experiment
The stabilization of soil organic matter (SOM) is an important process in the context of global change and is strongly affected by soil use and management. We investigated the mineralization and stabilization of 14C-labeled wheat (Triticum aestivum L.) straw and 14C-labeled farmyard manure under different cropping systems (crop rotation, monoculture, and bare fallow) in a long-term field experiment established in 1967. Exponential decay equations were fitted to the measured data, and coefficients of the C pools and decay constants were estimated. The different organic amendments and cropping systems resulted in considerably different distributions and half-lives of three modeled SOM pool compartments. Significantly lower amounts of residual 14C-labeled material were found in the bulk soils of the bare fallow systems than the farmyard-manured crop rotation or monoculture, indicating enhanced organic matter stabilization in the latter systems. In 2004, remaining 14C-labeled amendments (as a percentage of the original application in 1967) in bulk soils were 8.7 ± 0.3% for crop rotation, 8.6 ± 0.4% for monoculture, 7.5 ± 0.7% for bare fallow (all farmyard-manured), and 5.0 ± 0.6% for straw-manured bare fallow. Humic acids extracted with 1 mol L–1 NaOH responded markedly to the different cropping systems regarding both their extraction yields and the remaining 14C label. In all cropping systems, the 14C in the extracted humic acids decreased rapidly with time. Remaining 14C-labeled organic residues in this fraction in 2004 (as a percentage of the original application in the fraction) were 0.315 ± 0.002% for crop rotation, 0.31 ± 0.02% for monoculture, and 0.16 ± 0.02% for bare fallow (all farmyard manured). Our study therefore shows that the investigated humic acids are a dynamic SOM pool cycling at decadal and subdecadal rates.