At each site, twelve 4.75 x 9.14 m plots were established on flat, relatively uniform fields (Figs. A1, A2). Three plots were randomly assigned to be control plots, and three plots each received 0.64 cm, 1.3 cm, or 2.5 cm of compost which corresponds to approximately 0, 3.2, 6.4, 12.8 kg m2, respectively, which spans added amounts in previous studies (Kutos et al. 2022). Baseline monitoring was completed just prior to compost addition in September 2020 at each site and annually thereafter.
In each plot, two 1 m2 quadrats for aboveground biomass clip plots were marked 1 m from the edge of the narrow axis of the plot to avoid edge effects. One of the quadrats was surrounded by 4 ft tall fencing to create an exclosure within each plot. Two 5 m long transects for vegetation functional group composition were marked 1 m from the plot edges and 1 m from the clip plots.
Soil cover by plant functional group was recorded along 50 points on each transect and categorized as living vegetation, litter (included fine and coarse herbaceous and woody debris and dung) and bare ground (included soil and rock).
Aboveground biomass was collected in the exclosure and open quadrats. We used a 1 m2 grid divided into 100 cm2 squares and randomly selected without replacement ten squares to clip to ground level each year. Biomass was placed in paper bags, dried at 60C for 3d, and oxidized material was removed to capture material that was likely to have been alive in the previous 12 months. Material was weighed to 0.1 g. Belowground biomass in the top 10 cm was collected in the same squares in the 1 m2 grid as the aboveground biomass with 0.9 cm diameter soil core. Soils were sieved to 2 mm and roots were picked out from all fractions with forceps, soaked and washed, then dried for 60C for 3d and weighed to 0.001 g.
Bulk density was collected from 0-10 cm using a rigid metal cylinder (diameter 5.7 cm) hammered into the soil then excavated and placed in a plastic bag for transport to the lab to dry at 60C for 3d. Aggregate stability was measured on 6 haphazardly-collected surface samples using methods from Herrick et al. (2001). This method would not capture soils that could not be collected on the sieve (aka, category 0), but was consistent across all treatments. Infiltration rate was measured with a single ring infiltrometer (15.24 cm diameter) pounded into the soil in a randomly selected interspace (without perennial plants inside) to 10 cm depth. 444 mL of water was added and the time for all soil to infiltrate until soil surface was recorded. A second 444 mL was added and the time for the water to infiltrate was recorded, up to 30 min. Soil from the aggregated belowground biomass sample outside the exclosure was used to determine organic carbon by combustion method (Ward Labs, IA).