Else K. Bünemann

Impact of agricultural inputs on soil organisms—a review

External agricultural inputs such as mineral fertilisers, organic amendments, microbial inoculants, and pesticides are applied with the ultimate goal of maximising productivity and economic returns, while side effects on soil organisms are often neglected. We have summarised the current understanding of how agricultural inputs affect the amounts, activity, and diversity of soil organisms. Mineral fertilisers have limited direct effects, but their application can enhance soil biological activity via increases in system productivity, crop residue return, and soil organic matter. Another important indirect effect especially of N fertilisation is soil acidification, with considerable negative effects on soil organisms. Organic amendments such as manure, compost, biosolids, and humic substances provide a direct source of C for soil organisms as well as an indirect C source via increased plant growth and plant residue returns. Non-target effects of microbial inoculants appear to be small and transient. Among the pesticides, few significant effects of herbicides on soil organisms have been documented, whereas negative effects of insecticides and fungicides are more common. Copper fungicides are among the most toxic and most persistent fungicides, and their application warrants strict regulation. Quality control of organic waste products such as municipal composts and biosolids is likewise mandatory to avoid accumulation of elements that are toxic to soil organisms.

Phosphorus in Action

Soil organic phosphorus speciation by spectroscopic techniques.- Characterization of phosphorus forms in soil microorganisms.- The use of tracers to investigate phosphate cycling in soil/plant systems.- Molecular approaches to study biological phosphorus cycling.- Modelling phosphorus dynamics in the soil-plant system.- Role of mycorrhizal symbioses in phosphorus cycling.- Solubilization of phosphorus by soil microorganisms.- Role of soil macrofauna in phosphorus cycling.- Role of phosphatase enzymes in soil.- Rhizosphere processes, plant response and adaptations.- Biological phosphorus cycling in grasslands - Interactions with N.- Biological phosphorus cycling in arctic and alpine soils.- Phosphorus nutrition of forest plantations: the role of inorganic and organic phosphorus. - Phosphorus cycling in tropical forests growing on highly weathered soils.- Biological P cycling in dryland regions.- Manure management effects on phosphorus biotransformations and losses in animal production.- Management impacts on biological phosphorus cycling in cropped soils.- Phosphorus and global change.

The Use of Tracers to Investigate Phosphate Cycling in Soil-Plant Systems

The use of tracers is relevant to study the transformations of phosphorus (P) in the soil–plant system because (a) only a small fraction of the total soil P is rapidly circulating in this system, (b) P participates in many reactions in the soil, some occurring within a few seconds, others over years, and (c) P is distributed in many pools in the soil. This review presents the use of P radioisotopes (a) to probe pools and to study P transformations in soils, (b) to trace the fate of fertilizers in soil–plant systems, and (c) to assess the foraging strategies of arbuscular mycorrhizal fungi for P. Finally, we discuss the potential of analyzing the oxygen isotopes bound to P to study soil P dynamics and the research needed to achieve this aim.

Oxygen isotopes unravel the role of microorganisms in phosphate cycling in soils

Phosphorus (P) is considered the ultimate limiting nutrient for plants in most natural systems and changes in the distribution of inorganic and organic P forms during soil development have been well documented. In particular, microbial activity has been shown to be an important control on P cycling but its contribution in building up the pool of plant-available P during soil development is still poorly quantified. To determine the importance of different biological processes on P cycling, we analyzed the isotopic composition of oxygen in phosphate (δ(18)O-Pi) from the parent material, soil microorganisms, the available P pool, and from the vegetation along a 150-year soil chronosequence of a glacier forefield. Our results show that at all sites, δ(18)O-Pi of microbial Pi is within the range expected for the temperature-dependent equilibrium between phosphate and water. In addition, the isotopic signature of available Pi is close to the signature of microbial Pi, independently of the contribution of parent material Pi, vegetation Pi or Pi released from organic matter mineralization. Thus, we show that phosphate is cycled through soil microorganisms before being released to the available pool. This isotopic approach demonstrates for the first time in the field and over long time scales, and not only through controlled experiments, the role of the microbial activity in cycling of P in soils.

Long-term effects of crop rotation, stubble management and tillage on soil phosphorus dynamics

The effects of various management practices on soil phosphorus (P) dynamics were investigated in a field experiment in New South Wales, Australia, during 24 years of different crop rotation, stubble management, and tillage treatments. Topsoil samples collected at the beginning of the trial and after 6, 12, 18, and 24 years were analysed for resin-extractable P, inorganic and organic P, and total P. According to the calculated P input–output budget, 9–14 of the 20 kg P/ha added as superphosphate annually remained in the system, depending on the treatment. The measured increase in total P in 0–0.20 m did not differ between treatments, showing an accumulation rate of only 9 ± 2 kg P/ha.year. These results suggest a loss of 4 ± 2 kg P/ha.year, presumably into lower soil layers. Resin-extractable P at 0–0.10 m increased by 1.7 kg P/ha.year, irrespective of the treatment. The increase in total P after 24 years was almost completely accounted for by the increase in total extractable inorganic P. Changes in organic P paralleled changes in organic carbon, with a significant loss in treatments with stubble burning (wheat–lupin rotation and continuous wheat), and a significant accumulation in a wheat–subterranean clover rotation with stubble retention and direct drilling. We conclude that on the time scale of this experiment, the dynamics of carbon and organic P are closely linked.

Increased availability of phosphorus after drying and rewetting of a grassland soil: processes and plant use

AimsDrying and rewetting (DRW) often increases soil phosphorus (P) availability. Our aims were to elucidate underlying processes and assess potential plant uptake of released P.MethodsUsing a grassland soil with low available and high microbial P as a model, we studied the contributions of microbial and physicochemical processes to P release by determining DRW effects on i) C:P ratios of nutrient pulses in fresh and sterilized soils, ii) aggregate stability and iii) P forms released upon soil dispersion. Use of the P pulse by maize was examined in a bioassay and a split-root experiment.ResultsThe strong P pulse after DRW was larger than that observed for C. Experiments with sterilized soil pointed to a non-microbial contribution to the pulse for P, but not for C. Aggregate disruption after DRW occurred due to slaking, and this released molybdate-reactive and -unreactive P. Maize benefitted from the P pulse only in the bioassay, i.e. when planted after the DRW cycle.ConclusionsThe majority of C and P released upon DRW originated from the microbial biomass, but for P release, physicochemical processes were also important. In the field, the released P would only be available to drought-resistant plants.

Characterization of Phosphorus Forms in Soil Microorganisms

Characterization of phosphorus (P) forms in soil microorganisms is a novel approach to reach a better understanding of the role of bacteria and fungi as sink and source of P. After an overview of methods for cultivation of microorganisms, extraction from soil, and chemical analysis, two case studies are presented, one on pure cultures and one on microbial cells extracted from soil. Analysis of pure cultures of bacteria and fungi by 31P NMR suggested a predominantly fungal origin of pyrophosphate, polyphosphate, and phosphonates in soils. The first report of P forms in microbial cells extracted from soil showed similar concentrations of total P, P in phospholipids, and DNA per cell as found in aquatic microorganisms, but lower concentrations of RNA. Cell P concentrations tended to increase upon carbon addition to a tropical Ferralsol, whereas sole or additional P amendment had no significant effect. The scope and limits of this new approach are discussed.

phoD Alkaline Phosphatase Gene Diversity in Soil

ABSTRACT Phosphatase enzymes are responsible for much of the recycling of organic phosphorus in soils. The PhoD alkaline phosphatase takes part in this process by hydrolyzing a range of organic phosphoesters. We analyzed the taxonomic and environmental distribution of phoD genes using whole-genome and metagenome databases. phoD alkaline phosphatase was found to be spread across 20 bacterial phyla and was ubiquitous in the environment, with the greatest abundance in soil. To study the great diversity of phoD, we developed a new set of primers which targets phoD genes in soil. The primer set was validated by 454 sequencing of six soils collected from two continents with different climates and soil properties and was compared to previously published primers. Up to 685 different phoD operational taxonomic units were found in each soil, which was 7 times higher than with previously published primers. The new primers amplified sequences belonging to 13 phyla, including 71 families. The most prevalent phoD genes identified in these soils were affiliated with the orders Actinomycetales (13 to 35%), Bacillales (1 to 29%), Gloeobacterales (1 to 18%), Rhizobiales (18 to 27%), and Pseudomonadales (0 to 22%). The primers also amplified phoD genes from additional orders, including Burkholderiales, Caulobacterales, Deinococcales, Planctomycetales, and Xanthomonadales, which represented the major differences in phoD composition between samples, highlighting the singularity of each community. Additionally, the phoD bacterial community structure was strongly related to soil pH, which varied between 4.2 and 6.8. These primers reveal the diversity of phoD in soil and represent a valuable tool for the study of phoD alkaline phosphatase in environmental samples.

Management Impacts on Biological Phosphorus Cycling in Cropped Soils

Phosphorus (P) is a limited resource and P deficiency limits crop production on large areas worldwide. Future food security, therefore, will largely depend on efficient P use in cropping systems. In this review, we present the impact of farmers’ interventions on biological P cycling in cropped soils of temperate and tropical regions, with emphasis on microbial functions in soil P dynamics. We exemplify the effects of (1) soil tillage, with a focus on the comparison of conventional tillage versus direct seeding systems; (2) fertilizer input, using organic and/or mineral nutrient sources; and (3) integration of legumes into cropping systems. We analyze whether and how biological processes can be influenced to increase the use efficiency of soil and fertilizer P. Finally, we formulate recommendations for an integrated P management. Future research should target improved biological access to recalcitrant inorganic and organic P forms.

Phosphorus and Sulphur Cycling in Terrestrial Ecosystems

Phosphorus (P) and sulphur (S) are essential elements for all living cells. Among the biomolecules that contain P are nucleic acids (DNA and RNA), phospholipids, sugar phosphates (e.g. glucose-6-phosphate) and molecules with an energy-rich pyrophosphate bond (e.g. ATP), whereas S is contained in two amino acids (cysteine and methionine) and various coenzymes, vitamins and sulpholipids. The forms, amounts, transformation processes and cycling rates of the two elements in terrestrial ecosystems are usually studied either from an agronomic point of view, i.e. from the perspective of imminent deficiencies, since both elements are major plant nutrients and therefore essential to achieve sufficient crop yields, or from an environmental point of view, where a surplus of these elements in ecosystems may lead to eutrophication or even direct toxicity effects in the case of S.