A 1915–2011 microscale record of soil organic matter under wheat cultivation using FTIR-PAS depth-profiling

Abstract

Soil organic matter (SOM) is a keystone soil property that influences soil biological, chemical, and physical properties important for soil health. Due to the high fraction of carbon within SOM, soil organic C is an important and readily manageable component of the global carbon cycle. Agroecosystem management practices strongly influence SOM content and structural chemistry. However, there are few sites where long-term effects, e.g., more than 100 years, of agroecosystem management on SOM can be studied. We hypothesized that long-term wheat production and changes in residue management would alter SOM structural chemistry through time and space. Here, soil samples from years 1915, 1938, 1962, 1988, and 2011 were acquired from the continuous wheat with inorganic fertilizer application plot (plot 2) established in the year 1888 at Sanborn Field, Columbia, Missouri, USA. Control soil samples were collected in the year 2011 from a nearby native prairie. SOM structure was analyzed as a function of distance from the particle surface, at the micrometer scale, by Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS). Our results show that SOM profiles are very heterogeneous, but three different layers were identifiable in spectra collected using FTIR-PAS depth-profiling as follows: (1) a surface layer, ranging 0–20 μm, that exhibits enrichment of C–O bonds, (2) a middle layer, ranging 20–40 μm, displaying an abundance of aromatic C = C bonds, and (3) an inner layer, ranging 40–100 μm, containing more heterocyclic N moieties. Our findings show that transformation of SOM was accelerated during the first 50 years of wheat cropping and fertilization, especially in the surface layer. However, further changes in SOM structure were remarkably retarded between 1962 and 2011 due to a change in cropping practice that retained crop residue on the plot.