DOI: 10.25829/bexis.26786-8
title: Infiltration capacity and hydraulic conductivity of 50 grassland soils with varying land-use intensity and biodiversity in southern Germany (Schwäbische Alb) in 2017

description: Up to now only little attention has been given to the role of biodiversity for soil erosion and flood generation. Both, soil erosion and flood generation are driven by the capability of soils to take up (infiltration) and conduct water to deeper soil layers (hydraulic conductivity). A high infiltration capacity (i.e., maximum infiltration rate under a given matrix potential) reduces the risk of surface runoff and thus soil erosion. A high hydraulic conductivity facilitates the storage of water in the deeper soil, which mediates the time until rainwater reaches groundwater or a stream and thus affects flood generation. The main objective of our study was to determine if biodiversity affects the infiltration capacity (vB) and the saturated (K*) and unsaturated (Ku) hydraulic conductivity near saturation of soils in established and agriculturally managed grassland systems.
For this, we measured vB with a hood infiltrometer at several matrix potentials on 50 grassland plots along a land-use intensity gradient in the Biodiversity Exploratory Schwäbische Alb, Germany,  and calculated the saturated and unsaturated K. The studied plots were agriculturally managed grasslands with varying biodiversity and their size was 50 m x 50 m (i.e. the 50 grassland EPs of the Exploratory Schwäbische Alb). The measurements were done between 26 September and 20 October 2017.
The work has been funded by the DFG Priority Program 1374 ”Infrastructure-Biodiversity-Exploratories”and the Stiftung Landesbank Baden-Württemberg.
description: Study Design: Between 26 September and 20 October 2017, we measured the infiltration capacity near saturation once on each of the 50 grassland plots (EPs, see Fischer et al., 2010) in the Exploratory Schwäbische Alb in-situ with a hood infiltrometer. On one randomly selected plot (AEG23), we performed three separate measurements along a transect to check representativeness of the infiltration measurements. The three measured values for vB at psiM = -0.1 cm ranged between 9.3 and 36.3 10^−6 m/s. Compared to the range of all vB measurements at psiM = -0.1 cm (1.7 - 433.1 10^−6 m/s), the range of the measurements on AEG23 is small (only 6% of the range of all plots) and thus, we considered a single measurement per plot to be sufficient.|Sampling design: We measured the infiltration capacity near saturation per plot in-situ with a hood infiltrometer (UGT, Müncheberg, Germany; method described in Schwärzel and Punzel (2007)). We chose this method because it does not disturb the soil or damage the measurement site (like bore holes for example), which was of particular importance for the plots located in a nature reserve and for preservation of the plots for the long-term Biodiversity Exploratories project. The measurements were carried out using a hood with a radius of 11.4 cm which was placed on the soil surface inside a metal ring with a radius of 12.4 cm. The vegetation within the metal ring was cut to 1-2 cm height. The space between the ring and the hood was sealed with wet fine sand. Subsequently, the hood was flooded and the hydraulic pressure head in the water volume under the hood was controlled via a Mariotte water supply system. The applied pressure head, i.e. matrix potential (psiM) or tension (h), was measured using a U-tube manometer. The steady-state infiltration was recorded after three constant readings from the infiltration reservoir of the Mariotte water supply system. At psiM = 0 cm, the soil is saturated and all pores can participate in water flow, while at psiM = -2.0 cm the largest active pores can be estimated to have an approximate equivalent diameter of 1.5 mm, which further decreases to approximately 0.75 mm at psiM = -4 cm (Jarvis et al., 1987). On each plot, we measured the infiltration at the matrix potentials psiM = -0.1 and -2.0 cm. The matrix potential was then further decreased in 2 cm steps and infiltration measured until the bubble point of the soil was reached. The bubble point of the soil represents the matrix potential at which air permeates through a pore channel into the hood, i.e. the minimum matrix potential that can be applied at the respective site. On a few plots with especially slow infiltration and high bubble point, we did not measure all matrix potential steps up to the bubble point.|Data preparation: For the equations for the calculation of Ku, K*, and nuB (or vB) see the manual of the hood infiltrometer or Leimer et al. (2021). In brief, the infiltration capacity at a specific matrix potential is directly related to the hydraulic conductivity (Wooding, 1968). Although, it can be argued that the assumptions of the Richards equation are not met in natural soils, we calculated the saturated (K*) and unsaturated (Ku) hydraulic conductivity according to Wooding (1968). For this purpose, Gardner’s coefficient alpha was calculated from measured infiltration capacities vB at two different, adjacent matrix potentials (psii element of {-0.1, -2.0, -4.0, -6.0 cm}) at the same location. The unsaturated hydraulic conductivity was then calculated per matrix potential using the radius of the infiltration surface (a = 12.4 cm). The saturated hydraulic conductivity was finally calculated according to Gardner (1958) using psi = -0.1 cm, Ku and alpha as calculated from measurements at psi = -0.1 cm and -2.0 cm. On one plot (AEG21) the measurement at psi = -2.0 cm and at one plot (AEG19) the measurement at psi = -4.0 cm had to be discarded. Therefore, K* and Ku at psi = -0.1 cm on AEG21 were calculated from the measurements at psi = -0.1 cm and -4.0 cm and Ku at psi = -2.0 cm on AEG19 from measurements at psi = -2.0 cm and -6.0 cm. Accordingly, there are no values for vB and Ku at psi = -2.0 cm for AEG21 and at psi = -4.0 cm for AEG19.