The emission of nutrients and pesticides from agricultural soils endangers natural habitats. Here, we review to which extent carbon-rich organic amendments help to retain nutrients and pesticides in agricultural soils and to reduce the contamination of surrounding areas and groundwater. We compare straw, compost, and biochar to see whether biochar outperforms the other two more traditional and cheaper materials. We present a list of criteria to evaluate the suitability of organic materials to be used as soil amendments and discuss differences in elemental compositions of straw, compost, and biochar to understand, how soil microorganisms utilize those materials. We review their effects on physical and chemical soil characteristics, soil microbial communities, as well as effects on the transformation and retention of nutrients and pesticides in detail. It becomes clear that for all three amendments their effects can vary greatly depending on numerous aspects, such as the type of soil, application rate, and production procedure of the organic material. Biochar is most effective in increasing the sorption capacity of soils but does not outperform straw and compost with regards to the other aspects investigated. Nevertheless, the possibility to design biochar properties makes it a very promising material. Finally, we provide critical comments about how to make studies about organic amendments more comparable (comprehensive provision of material properties), how to improve concepts of future work (meta-analysis, long-term field studies, use of deep-insight microbial DNA sequencing), and what needs to be further investigated (the link between structural and functional microbial parameters, the impact of biochar on pesticide efficiency).

With this study, we aim to relate the substrate quality of different organic materials derived from plant residues to the respiratory activity of soil microorganisms after amendment, the formation of oxygen gradients upon irrigation, and the leaching of macronutrients and metals in soil. Elemental analyses were performed to determine the chemical composition of wheat straw, green compost, and a biochar product, showing that carbon availability, C/N ratio, and metal contents varied markedly. Consequently, after application to well-aerated sandy loam soil at 1% w/w, only straw increased microbial activity substantially, and nitrate was depleted within one week. Upon intense irrigation of soil columns packed with differently amended soils, strong hypoxia formed only in straw–soil, where microbial oxygen demand for straw degradation was high. This was enhanced after the application of mineral fertilizers, and nitrate leaching was mitigated. With the decreasing redox potential in straw–soil, the leaching of Fe, Mn, Al, Ni, Co, and As was increased. However, nitrate from mineral fertilizer mitigated the reduction of redox potential and, thus, the leaching of these metals. Measuring oxygen at different depths revealed near anoxic conditions at −15 cm of straw–soil with NP-fertilizer applied within 12 h after the start of irrigation and remained for at least 60 h, while oxygen showed extensive fluctuations in the upper few centimeters. This study showed that organic soil amendments with high carbon availability induce microbial respiration to the extent that causes strong and long-lasting hypoxia upon irrigation, even in sandy soil, which leads to substantial effects on the mobility of nutrients and toxic metals. In contrast, organic soil amendments with low carbon availability did not cause such effects.

The structural response of bacterial and fungal soil communities to four carbon-rich organic amendments of increasing recalcitrance was investigated. Wheat straw, green compost, a mixed product based on biogas residues, and a fermented biochar were applied to a sandy agricultural soil of low organic carbon content. After laboratory incubation for 6 months, the community structure was investigated via DNA sequencing. All amendments caused changes in the communities of bacteria and fungi, but to different extents, with the communities exposed to more recalcitrant amendments showing the least variation compared to the non-amended soil. Changes in species composition as well as their relative abundances were observed. While the straw had a pronounced effect on bacteria (e.g., the highest number of indicator species), effects of the composted, fermented, or pyrolyzed materials were minor. Hierarchical clustering showed that the fungal communities were more different from each other than the bacterial ones with the straw-soil being most different and the biochar-soil least different from the non-amended soil. While the abundant fungal species in biochar-soil and non-amended soil were very alike, especially rare fungal species shifted upon addition of biochar. An indicator species analysis identified specific taxonomic groups which were triggered by the different organic materials. We conclude that bacterial and fungal communities strongly change upon input of degradable carbon (straw), while fungi in particular respond to the application of processed organic materials. With this study, we report the consequences of applying organic materials for the microbial community in one soil. We provide these data for meta-analyses that are required to unravel all relevant interactions across different soils, organic materials, and time. This will allow to better understand and predict the effects of organic soil amelioration measures on soil microorganisms.

Current knowledge about how biochars affect the fate of pesticides in soil is based on studies that used pure biochars. After finding that an additional biological post-pyrolysis treatment, such as co-composting or lactic fermentation, is required for biochars for superior performance in temperate arable soils, a knowledge gap formed of how such further processed biochar products would affect the fate of pesticides in soil. This study compared the effects of a novel fermented biochar alone or mixed with biogas residues on the fate of two pesticides, 4-chloro-2-methylphenoxyacetic acid (MCPA) and metalaxyl-M, in a temperate arable soil to the traditional organic amendments wheat straw and compost. The fate of ¹⁴C-labeled MCPA was markedly affected in different ways. Fermented biochar effectively reduced the water-extractability and mineralization due to adsorption that was comparable to adsorption strengths reported for pure biochars. However, this effect was weak for the biochar mixed with biogas residues. Straw reduced water-extractable amounts due to increased biodegradation and formation of likely biogenic non-extractable residues of MCPA. In contrast, compost decelerated mineralization and increased the water solubility of the MCPA residues due to released dissolved organic matter. The amendments' effects were minor regarding ¹⁴C-metalaxyl-M, except for the fermented biochar which again reduced water-extractability and delayed degradation due to adsorption. Thus, the effects of the organic amendments differed for the two pesticide compounds with only the fermented biochar's effect being similar for both. However, this effect was no longer present in the mixed product containing 20% biochar. Our findings clearly show that biologically treated biochar-containing products can affect the fate of pesticides in soil very differently, also when compared to traditional organic amendments. Such impacts and their desirable and undesirable ecotoxicological implications need to be considered before the large-scale application of biochars to temperate arable soils.

Background: Urease inhibitors and nitrification inhibitors delay nitrogen fertilizer transformations in soil to reduce nitrogen losses and increase nitrogen use efficiency. While new inhibitor compounds are constantly being developed, little is known about non-target effects on the soil microbiome. This is the first study to investigate non-target effects of the urease inhibitor 2-NPT and the nitrification inhibitor MPA on the soil microbiome. In addition, the more established nitrification inhibitor DMPP was investigated. Target effects and potential non-target effects on the function and composition of microbial communities in three soils from Germany were assessed. Results: Soil microcosms were treated with practically relevant doses of inhibited and non-inhibited fertilizer products. Effects on soil nutrients and enzyme activities from nutrient cycling were analyzed. Gene abundances of bacterial and archaeal amoA as well as 16S rRNA and ITS marker genes were quantified using the QIAcuity nanoplate digital PCR. The bacterial and fungal community compositions were analyzed via amplicon sequencing of 16S rRNA and ITS marker genes. Significant reductions of target enzyme activities were found for 2-NPT and DMPP but not MPA. Effect size of inhibition was soil-dependent. Ammonium and nitrate concentrations were significantly affected by the inhibitors in one of the three soils. The non-target enzyme activities of phosphatase, beta-glucosidase, and arylsulfatase were not affected by the inhibitors. Both nitrification inhibitors primarily targeted bacterial ammonia oxidizers, as bacterial but not archaeal amoA genes were reduced. Overall bacterial and fungal communities were not clearly affected. Observed abundance shifts of soil microorganisms were linked to indirect effects driven by nutrient availability rather than direct effects of the inhibitors. Conclusion: Our study suggests that the newer inhibitors 2-NPT and MPA as well as DMPP do not directly affect the function and composition of the soil microbiome in the short-term. Significant target effects of the inhibitors change the availability of mineral N which causes indirect effects with minimal non-target microbial community shifts. Further tests involving other soil organisms and long-term field studies are required to further improve the environmental risk assessment of inhibitors.