Probing deep to express root‐zone enrichment of soil‐test biological activity on southeastern U.S. farms

Abstract

Soil organic carbon (C) and nitrogen (N) accumulation contributes to improved soil health condition, particularly after a history of tillage‐intensive land use. Soil‐test biological activity (STBA) is an active fraction of organic matter that is responsive to conservation management. This essay summarizes the need, concept, and method of calculating root‐zone enrichment of STBA and other organic C and N fractions on private farms. Calculation of root‐zone enrichment separates the pedogenic influence on organic matter content from that of contemporary management. This separation is particularly important when attempting to determine STBA or soil organic C stock change in response to management across variable landscapes. Reasonable farm‐level estimates of STBA and stocks of soil organic C and N can be obtained from one to two dozen sampling sites on farms with differences in land use, a process that could help propel in‐depth assessments of soil health condition and C stock change.

Root‐zone enrichment of soil‐test biological activity and particulate organic carbon and nitrogen under conventional and conservation land management

“…The positive association, albeit relatively weak, between root-zone enrichment and baseline STBA suggests that the quantification of antecedent condition at 30-cm depth will be important to obtain more reliable estimates of rootzone enrichment of STBA. However, it can also be stated that root-zone enrichment of STBA varies relatively little across a diversity of baseline soil conditions compared with much greater land-use effects, as noted previously (Franzluebbers, 2021b(Franzluebbers, , 2022a.…”

“…Soil-test biological activity is determined from the flush of CO 2 emitted from microbial activity during the first 3 days of incubation following rewetting of dried soil. Significant root-zone enrichment of STBA has been documented in conservation-tillage cropping systems in North Carolina (Franzluebbers, 2021a), and limited evidence suggests that conservation management with grazed pastures and woodlands could lead to greater accumulation of STBA (Franzluebbers, 2022a). However, broad applicability of root-zone enrichment assessments of STBA, POC, and PON on different soils has not been investigated.…”

Should we consider integrated crop–livestock systems for ecosystem services, carbon sequestration, and agricultural resilience to climate change?

“…Calculation of root‐zone enrichment of soil organic C and N is relatively new but is an important consideration for on‐farm soil health analyses that can vary in soil types and landscape features. Root‐zone enrichment attempts to separate contemporary management influences from those of pedogenic origin in a conceptual framework implemented with standard calculations (Franzluebbers, 2022). This approach assumes (1) pedogenesis will have a large overriding influence on baseline condition of soil organic C and N; (2) when historical management has caused extensive erosion, residual surface organic C and N concentrations would be low and more like that of the subsoil; (3) bulk density of soil throughout the profile is considered inversely proportional to soil organic C concentration; and (4) soil organic C and N concentrations at selected sampling depth intervals can reasonably characterize actual depth distribution and be fitted to a nonlinear function.…”

“…Root‐zone enrichment was calculated from a nonlinear regression of soil organic C or N concentration by depth increment of each management condition. Details of the calculation approach were presented elsewhere (Franzluebbers, 2022) but can be summarized by the following set of equations: $$$$\begin{equation}{\text{Soil organic C stock}} = \sum {{\mathrm{B}}{{\mathrm{D}}_i} \times {\mathrm{SO}}{{\mathrm{C}}_i}} ,\end{equation}$$$$ $$$$\begin{eqnarray}{\text{Baseline soil organic C stock}} &=& {\mathrm{SO}}{{\mathrm{C}}_{30{\mathrm{ {\hbox {-}}cm\ depth}}}}\nonumber\\ && \times\ {\mathrm{B}}{{\mathrm{D}}_{30{\mathrm{ {\hbox{-}}cm\ depth}}}} \times 30,\end{eqnarray}$$$$where BD i is bulk density (Mg m −3 ) of each 5‐cm depth increment from the surface to 30‐cm depth, and SOC i is soil organic C (g kg −1 ) of each 5‐cm depth increment from the surface to 30‐cm depth. Incremental concentrations were calculated as the means from the top and bottom points, for example, SOC at 0‐cm depth and 5‐cm depth using the nonlinear regression equation.…”

“…Multiple fields under three long‐term land uses were sampled at three locations in North Carolina (Southern Blue Ridge Major Land Resource Area [MLRA] in Haywood County, Southern Piedmont MLRA in Rutherford County, and Coastal Plain MLRA in Halifax County) (Table 1). This sampling effort was described in Franzluebbers (2022) for the assessment of soil‐test biological activity, but data for total organic C and N had not been reported before. A key observation from this sampling effort was the unique interpretation of land‐use effect when considering root‐zone enrichment of soil organic C compared with total stock of soil organic C. Across locations, the total stock of soil organic C was not different between cropland and grassland but was greater under woodland than under grassland.…”