DOI: 10.5061/dryad.9w0vt4bk0
title: The influence of inherent soil factors and agricultural management on soil organic matter

description: The accumulation of soil organic matter (SOM) is vital to the agronomic
 and environmental functioning of agroecosystems, yet the relative
 influence of inherent soil properties and agricultural management
 practices on SOM dynamics are not often addressed in individual studies.
 Using a network of 218 operating farm fields across Wisconsin and southern
 Minnesota, USA, this research employs single variable analysis (ANOVA and
 regression) and regression tree analysis to assess the effects of soil
 properties (texture, drainage class, pH) and management variables related
 to crop rotation, tillage, cover cropping, and manure application on SOM,
 as well as total organic carbon (TOC) and total nitrogen (TN) in the upper
 15 cm. Single variable analysis revealed that greater SOM, TOC, and TN
 were associated with poorly drained soil, tile-drained fields, high-clay
 content soil, and high biomass crop rotations. Soil organic matter (SOM)
 and TOC were strongly related (R2=0.71), but different regression trees
 were produced; SOM was most influenced by clay content, while TOC was most
 influenced by drainage class. Future assessment for the building of SOM or
 TOC should be conducted with drainage and texture class categories and on
 a regional basis, given that these factors influence the practices that
 occur within landscapes. A rapid building of data sets through
 unstructured sampling, including an abundance of meta-data, should be a
 research priority in agricultural science to identify practices to build
 SOM on a regional basis.
description: Field descriptions and
 sampling. 
 Soil samples were collected
 from 218 farm fields across Wisconsin (n=212) and Minnesota (n=6) (Fig. 1)
 between 2015 and 2017. The fields represent a range of cropping systems
 common in the Upper Midwest. Six distinct regions were sampled and
 identified by either general region of a state (northeast Wisconsin,
 southeast Wisconsin, and southern Minnesota) or by watershed (Dry Run, Elk
 Creek, Jersey Valley) (Fig. 1). Elk Creek and Jersey Valley exist within
 the Driftless Region, an area characterized by steep slopes and flash
 flood events. All fields were planted into corn the season soil samples
 were collected. In each field, three composite soil samples were collected
 that consisted of five 0- to 15-cm soil cores collected with a probe of
 2.5- or 7.5-cm internal diameter. Most soil samples (194) were collected
 prior to fertilizer application and corn planting (mid-April); 24 samples
 were in late June (2017 only). Soil sampling was conducted with an area of
 36 m<sup>2</sup> within the dominant soil map unit as
 identified by the USDA NRCS Web Soil Survey (Soil Survey Staff, 2019) and
 from an area identified by the farmer where average crop yields were
 obtained. The composite samples were stored cold and transferred into a
 freezer with 1 to 6 hours of sampling to stagnate microbial metabolism and
 organic matter mineralization. Within 30 days, soil samples were thawed
 and dried for 1 week at 32˚C in a forced-air drier, ground to pass through
 a 2-mm sieve, and stored at room temperature until
 analysis. Inherent soil properties such as texture class,
 sand and clay content of the surface horizon, and drainage class were
 obtained from the USDA NRCS Web Soil Survey (Soil Survey Staff, 2019).
 Agronomic management information regarding crop rotation, tillage
 practices, cover crop use, tile drainage, and manure and fertilizer
 applications were obtained directly from each farmer through an in-person
 interview. Long-term crop management practices were difficult to obtain
 for all farms; for example, it was difficult to get accurate information
 on how long a field had received manure. The dataset constructed uses
 recent cropping history (past 5 years) as a representation of specific
 management practices (that often have occurred much longer than just the
 past 5 years). Based on the collected data, four categories for crop
 rotation (continuous corn, corn-soybean, corn with small grain, and corn
 with alfalfa) and five categories for previous crop were created
 (Supplementary Table 2). Two categorical data were developed for cover
 crops: if there was a cover crop planted last fall (yes or no) and the
 number of times a cover crop was planted in the past 5 years. Tillage
 practices were categorized by practice [no tillage, minimum tillage
 (including vertical tillage or strip tillage), and conventional tillage
 (chisel, disk or moldboard)] and by the number of tillage passes that
 occurred between harvest of the previous year’s crop and the planting of
 the current year’s crop (0 to 4). Tillage was only considered no-till or
 minimum tillage if practiced for more than 4 years. Manure was categorized
 based on the number of manure applications that occurred in the past 5
 years (0 to 5), when manure was applied in the past year (none, summer,
 fall, winter, or spring), and manure type (species and if solid or
 liquid). Tile drainage presence was also noted (yes or no). The manure N,
 fertilizer N, and total N input (which includes manure, fertilizer, and
 legume N inputs) (kg ha<sup>-1</sup>) to the previous corn
 crop were also collected. If farmers did not have manure analysis,
 estimates of available N were used (Laboski & Peters, 2012); N
 input from alfalfa biomass was assumed to be 101 kg
 ha<sup>-1</sup> (Laboski & Peters,
 2012).  Soil analysis.  Soil
 pH and SOM were analyzed by the University of Wisconsin Soil and Forage
 Analysis Laboratory (Marshfield, Wisconsin). Soil pH was calculated using
 a 1:1 slurry of 10 g soil and 10 mL of deionized water and measured with a
 glass electrode (Peters et al., 2015). Soil organic matter values were
 determined through loss on ignition by heating the soil to 360˚C for 2
 hours (Combs et al., 2015). Total C (TC) and total N (TN) levels were
 determined via the dry combustion method using a Flash EA 1112CN Automatic
 Elemental Analyzer (Thermo Finnigan, Milan, Italy). Between 8 to 10 mg of
 finely ground soil were packed into a 5 mm by 9 mm tin capsule prior to
 combustion at temperatures exceeding 1000°C. Soils with pH greater than
 7.0 were tested for effervescence using 5% HCl as an indicator if
 carbonates were present. If carbonates were not observed, TC was assumed
 to be TOC; if carbonates were observed, they were subject to
 acid-fumigation prior to dry combustion (Harris et al., 2001). Only 25
 samples were analyzed for carbonates and 13 of those had carbonate
 concentrations above the detection limit. There were 218 samples for SOM,
 but only 2016 for TOC and TN because two samples were accidently
 discarded.