Christopher Brock

The humus balance model (HU-MOD): a simple tool for the assessment of management change impact on soil organic matter levels in arable soils

With this paper we present a simple model for the assessment of management impact in arable farming systems on soil organic matter (SOM) levels. The humus balance model (HU-MOD) is designed for application by farmers and extension workers in practice as a tool for management support. To enable practice applicability, HU-MOD bypasses the need for data on soil parameters and can be run with simple management data. HU-MOD is based on a simplified model on carbon and nitrogen pools and fluxes in the soil–plant system. The model proved to be an applicable simple tool for the comparison of management systems in arable farming with regard to the impact on SOM levels. Even though an absolute quantification of SOM level changes is not possible due to the methodical approach bypassing the need for any data on soil parameters, the model may be used to assess a positive or negative impact of a management system or management period compared to a reference and thus may be used to assess the impact of management changes, or to analyse a specific impact for different management periods on a defined spatial unit.

Development of soil organic matter stocks under different farm types and tillage systems in the Organic Arable Farming Experiment Gladbacherhof

In 1998, the Organic Arable Farming Experiment Gladbacherhof (OAFEG) was started in order to explore the impact of different organic arable production systems (mixed farming, stockless farming with rotational ley, stockless cash crop farming) and of different tillage intensities (conventional plough as a full inversion tillage, two-layer plough, inversion tillage at reduced depth, non-inversion tillage) on sustainability parameters. In this article, we present results on the development of soil organic matter (SOM) levels. Starting with organic mixed farming with approximately 0.7 livestock units (LU) per ha cattle before set-up of the experiment, only the mixed farming system in the experiment was able to maintain SOM levels. The stockless system with ley maintained soil organic carbon (SOC), but lost soil total nitrogen (STN), and the stockless cash crop system had a significant SOM loss in the magnitude of 7.7 t SOM ha−1, or roughly 8.4% of the initial SOM mass. Reducing tillage intensity had no impact on SOM masses, but only on organic matter stratification in soils. We conclude that specialization of organic farms towards stockless arable crop production requires special attention on SOM reproduction to avoid detrimental effects. Further, reduced tillage intensity does not necessarily have a positive effect on SOM.

Soil organic matter balances in organic versus conventional farming—modelling in field experiments and regional upscaling for cropland in Germany

The question of whether organic farming leads to higher soil organic matter (SOM) levels in arable soils compared with conventional farming is an ongoing debate. Building on several studies reported in the literature, we hypothesize that the impact on SOM levels is not an intrinsic characteristic of any farming system but is the result of the actual structure of the farming system, in particular, the composition and management of crop rotations, and the availability and utilization of organic manure. The SOM balances for organic versus conventional farming in Germany are compared by considering data on the structure of organic and conventional farming systems from agricultural census reports and then applying the SOM balance model HU-MOD. Preliminary testing confirmed the applicability of the model using a survey on soil organic carbon (SOC) change and SOM balances in four long-term field experiments in Germany and Switzerland and found that more positive SOM balances coincided with higher SOC levels. We therefore conclude that, where the SOM supply level of organic farming systems is higher than in conventional management, a shift from conventional to organic agriculture would increase SOM levels. Upscaling using agricultural census data in Germany, we found that SOM balances of organic farming were more positive than for conventional farming in the scenarios without consideration of animal manure application, but SOM balances for the two systems were not different where animal manure application rates were assumed to be at the current average rate for all cropped land. However, in fact, animal manure availability and application shows strong regional variations, and it is likely that this would affect the mean cropland SOM balance if it were possible to calculate it based on such spatially disaggregated data. We confirm the applicability of simple SOM balance models to compare the impact of farming systems and cropland structures on SOC levels. More work is needed to develop data inputs at a sufficient spatial and structural resolution to support more detailed evaluation.

The role of soil depth in the evaluation of management-induced effects on soil organic matter: Soil depth and soil organic matter

&NA; The aim of the current survey was to determine the relevance of the subsoil in the assessment of management effects on soil organic matter (SOM) in arable farming. Data in this survey were provided by the organic arable long‐term field experiment Gladbacherhof (OAFEG). Three different ‘farming types’ were compared: mixed (MF), stockless with rotational ley (SFL) and stockless with cash crops (SFC). Each type had four different tillage treatments: full inversion (FIT), two‐layer plough (TLP), reduced inversion (RIT) and non‐inversion (NIT). The different mean masses of soil organic carbon (SOC) and soil total nitrogen (STN) at 0–30‐, 30–60‐, 60–90‐ and 0–90‐cm soil depths after three crop rotation cycles (17‐year observation period) were evaluated and considered as an indicator of SOM. ‘Farming types’ differ in their effect on SOC and STN mass of the topsoil layer (0–30 cm) in the order MF > SFL > SFC (P < 0.01). For the 0–90‐cm soil depth there were no differences between the treatments. Treatments with different tillage intensity did not show any effect on the contents of SOC or STN within the 0–30‐cm soil depth. However, within the 30–60‐cm depth full‐inversion tillage (FIT) was associated with larger amounts of SOC and STN than the reduced tillage intensity treatments TLP, RIT and NIT (P < 0.01). This situation even resulted in significantly larger SOC and STN masses under FIT (P < 0.05) over the whole soil profile under study (0–90 cm). Our results suggest that sampling depth should be extended to include the effects on the upper subsoil in assessment of the effects of arable management on soil organic matter. HighlightsRole of subsoil in the assessment of management effects on soil organic matter in arable farming is rarely accounted for.We investigated SOC and STN contents below 0–30‐cm depth in a long‐term field experiment.Consideration of the subsoil had a marked effect on the differentiation of the treatments.Sampling depth should be extended to include effects of management on the upper subsoil in the assessment of soil organic matter.

The effect of C:N ratios on the fate of carbon from straw and green manure in soil: The effect of C:N ratios on the fate of carbon from straw and green manure

&NA; Straw and green manures are among the most important soil amendments used worldwide to provide agricultural soil with organic matter (OM). The reported improvement in soil organic matter content varies greatly; therefore, there is a need for mechanistic approaches to evaluate substrate turnover in the soil. This study compares the results of a field and an incubation experiment to follow the fate of carbon from organic fertilizers in soil as a function of the C:N ratio of the amendments. The amendments were straw (small‐N, Hordeum vulgare L.; large‐N, Pisum sativum L.) or green manure (Sinapis alba (M)) with additional organic or mineral fertilizers to alter the C:N ratio of the soil‐substrate mixture. In the 100‐day field study, C respiration was measured at defined intervals at three locations with different soil types (Luvisol, Fluvisol and Arenosol). In the incubation experiment, C changes over 300 days were quantified based on the measurement of C respiration and C amounts in the soil‐substrate mixture at the start and end of the experiment. Furthermore, microbial biomass was quantified at the end of the experiment to enable an assessment of microbial C‐use efficiency. In the field experiment, a significant correlation between N supply and C fate occurred at one site only (Luvisol) where retention of C from organic substrates was larger with increased N supply. In the incubation experiment, N supply and C fate were correlated significantly for both soils under study (Arenosol and Fluvisol). Furthermore, C‐use efficiency was positively correlated with C retention from organic inputs to the Fluvisol in the incubation experiment, whereas the opposite situation occurred with the Arenosol. This indicated that C retention from organic inputs was not dependent on carbon‐use efficiency by the microbial community under these conditions. We conclude that stoichiometric implications apply to carbon retention in soil that depend on N availability for soil microbes. In the early stages of organic matter turnover, the outcome of N availability might be concealed by the effect of chemical recalcitrance of the substrates. The length of this phase will depend on environmental conditions for microbial activity. The effect of the C:N ratio on the fate of C from organic amendments must be considered in the assessment of C management strategies in arable farming. HighlightsOrganic inputs to soil with a large C:N ratio lose more C in turnover than amendments with a small C:N ratio.Retention of C from organic amendments in soil appears to depend on microbial C‐use efficiency and availability of nitrogen.The soil type and related environmental conditions affect CUE and the magnitude of C retention.Negative correlation between C:N ratio of organic amendments and C fate in soil must be considered in carbon management strategies.