Fungal biomass in pastures increases with age and reduced N input

Previous studies have shown that soil fungal biomass increases towards more natural, mature systems. Shifts to a fungal-based soil food web have previously been observed with abandonment of agricultural fields and extensification of agriculture. In a previous field experiment we found increased fungal biomass with reduced N fertilisation. Here, we explore relationships between fungi, bacteria, N input and grassland age on real dairy farms in the Netherlands. We hypothesised that also in pastures that are still in production there is a negative relationship between fungal biomass and fertilisation, and that fungal biomass increases with grassland age in pastures that are still in production. We expected the fungal/bacterial biomass ratio to show the same responses, as this ratio has often been used as an indicator for management changes. We sampled 48 pastures from eight organic dairy farms. Sites differed in age and fertilisation rate. We determined fungal and bacterial biomass, as well as ergosterol (a fungal biomarker). Fungal and bacterial biomass and ergosterol, showed a negative relationship with N application rate, and correlated positively with organic matter percentage. In old pastures, fungal biomass and ergosterol were higher than in younger pastures. Because bacterial biomass responded in the same way as fungal biomass, the F/B ratio remained constant, and can therefore—in our data set—not be used as an indicator for changing management. We conclude that the changes in fungal and bacterial biomass were driven by changes in organic matter quality and quantity. The negative relationship we found between N application rate and fungal biomass adds to earlier work and confirms the presence of this relationship in pastures with relatively small differences in management intensities. Earlier studies on shifts in fungal biomass focused on ex-agricultural fields or restoration projects. Here we show that fungal biomass is also higher in older agricultural pastures.     

Fungal/bacterial ratios in grasslands with contrasting nitrogen management

It is frequently hypothesised that high soil fungal/bacterial ratios are indicative for more sustainable agricultural systems. Increased F/B ratios have been reported in extensively managed grasslands. To determine the shifts in fungal/bacterial biomass ratio as influenced by grassland management and to find relations with nitrogen leaching potential, we sampled a two-year-old field experiment at an organic experimental farm in the eastern part of The Netherlands. The effect of crop (grass and grass-clover), N application rate (0, 40, 80, ) and manure type (no manure, farm yard manure and slurry) on the F/B ratio within three growing seasons was tested, as well as relations with soil and crop characteristics, nitrate leaching and partial N balance. Biomass of fungi and bacteria was calculated after direct counts using epifluorescence microscopy. Fungal and bacterial biomass and the F/B ratio were higher in grass than in grass-clover. The F/B ratio decreased with increasing N application rate and multiple regression analysis revealed a negative relationship with pH. Bacterial activity (measured as incorporation of [³H]thymidine and [¹⁴C]leucine into bacterial DNA and proteins) showed the exact opposite: an increase with N application rate and pH. Leaching increased with N application rate and was higher in grass-clover than in grass. Partial N balance was more positive at a higher N application rate and showed an inverse relationship with fungal biomass and F/B ratio. We conclude that the fungal/bacterial biomass ratio quickly responded to changes in management. Grasslands with higher N input showed lower F/B ratios. Grass-clover had a smaller fungal biomass and higher N leaching than grass. In general, a higher fungal biomass indicated a lower nitrogen leaching and a more negative partial N balance (or smaller N surplus), but more observations are needed to confirm the relationship between F/B ratio and sustainability.     

Organic resources and earthworms affect phosphorus availability to sorghum after phosphate rock addition in semi-arid West Africa

A field experiment was laid out in Burkina Faso (West Africa) on an Eutric Cambisol to investigate the interaction of organic resource quality and phosphate rock on crop yield and to assess the contribution of earthworms (Millsonia inermis Michaelsen) to P availability after phosphate rock application. Organic resources of different quality were applied at a dose equivalent to 40 kg N ha^–1 with or without phosphate rock from Kodjari (Burkina Faso) at a dose equivalent to 25 kg P ha^–1, and were compared with control and single phosphate rock treatments in a factorial complete block design with four replicates. Sorghum (Sorghum bicolor L. Moench) variety SARIASSO 14 was grown. Sheep dung had the highest impact on earthworm casting intensity followed by maize straw. Combining organic resources with phosphate rock reduced earthworm casting activities compared to a single application of organic resources or phosphate rock. Addition of phosphate rock to maize straw reduced P availability in earthworm casts whereas combining sheep dung or compost with phosphate rock increased P availability. The contribution of earthworms to Kodjari phosphate rock solubilisation mainly occurred through their casts, as the available P content of casts was 4 times higher than that of the surrounding soil.     

The influence of soil bulk density and soil moisture on the habitat selection of the dung beetleTyphaeus typhoeus in the Netherlands

Summary In the Netherlands, the dung beetleTyphaeus typhoeus (Col., Geotrupidae) is confined to sandy and loamy sandy soils. Experiments were carried out in the laboratory and in the field to ascertain the effects of soil bulk density and soil moisture on various aspects of the reproductive behaviour and development of this dung beetle. Some of the results were validated under natural field conditions. The nesting burrows were shown not to penetrate beyond the upper level of the soil moisture saturation zone. The depth of the burrows was not influenced by the bulk density in moist and free-drained sandy soils. Lower bulk densities of the soil were shown to result in fewer dung sausages being made, even when sufficient dung was availabe to provide for the larvae. This phenomenon was attributed to a behavioural response from the dung beetles. The critical bulk density in the field appeared to be approximately 1.40 × 10³ kg/m³. The survival rate of eggs and larvae was shown to be adversely affected by conditions of high soil moisture (> 20% by volume) at the depths where the larvae develop. Evidence is given for a low survival rate of larvae at conditions of low soil moisture (pF > 2.7) at these depths. The impact of certain other soil factors is discussed.Communication no. 280 of the Biological Station, Wijster, The Netherlands     

Recovery of nutrients from the liquid fraction of digestate: Use of enriched zeolite and biochar as nitrogen fertilizers

The liquid fraction after liquid/solid separation of biogas digestate has a high potential as a fertilizer due to its high nutrient concentration. However, the direct application of digestate in agricultural fields results in practical problems due to its voluminous nature. One solution to this could be to concentrate nutrients onto sorbents such as biochar or zeolites, which can subsequently be used as a fertilizer. This study investigated the ability of biochar and zeolite ‘clinoptilolite' enriched with digestate nutrients to supply nitrogen (N) when used as a fertilizer. A pot experiment with ryegrass was conducted to test the effect of a nutrient‐enriched biochar and clinoptilolite by determining plant biomass growth and N uptake. This included untreated biochar and clinoptilolite as controls and two levels of N application (15 and 45 mg N per pot) each at two initial loading ratios (low and high). Nutrient‐enriched biochar and clinoptilolite increased plant biomass yield (up to 1.02 and 2.39 g per pot) and N uptake (up to 11.23 and 39.94 mg N per pot) compared to the untreated sorbents treatments. Initial loading ratio had a significant effect on plant biomass response and apparent N recovery (ANR) for enriched clinoptilolite, and lower initial loading ratio improved plant growth. In contrast to clinoptilolite, higher initial loading ratio resulted in higher ANR. In conclusion, our results reveal that N released from enriched clinoptilolite and biochar could be taken up by the plants, clinoptilolite performed more effectively than biochar, and initial loading ratio affected the performance of the sorbents when used as a fertilizer.     

Soil fauna and soil function in the fabric of the food web

Over the last four decades, spanning David Coleman's career, and in no small measure thanks to him, soil ecologists have made tremendous progress in describing and understanding the overwhelming complexity of biological, biophysical and biochemical interactions in soil. These interactions shape the soil as a habitat for the soil food web and the vegetation and, thereby, regulate the two main life-supporting processes on Planet Earth: production and decomposition. Changes in decomposition and production processes are governed by (human-induced) changes in vegetation composition/cover, the amounts and quality of organic residues and (in)organic fertilizers entering the soil. Such modifications alter the physical environment and the soil biota. Hence, decomposition and production processes cannot be understood and/or manipulated without explicitly addressing the composition and activity of the soil food web. Using a conceptual model, we argue that quantitative understanding of biophysical interactions, in particular those between soil fauna and soil structure, are paramount to understanding biological and biochemical processes in soil and the availability of water and nutrients to plants. The need to increase the efficiency of crop production worldwide, to reverse soil degradation and to increase soil resilience will set the agenda for soil ecologists in the near future.     

Earthworm species composition affects the soil bacterial community and net nitrogen mineralization

Knowledge of the effects of species diversity within taxonomic groups on nutrient cycling is important for understanding the role of soil biota in sustainable agriculture. We hypothesized that earthworm species specifically affect nitrogen mineralization, characteristically for their ecological group classifications, and that earthworm species interactions would affect mineralization through competition and facilitation effects. A mesocosm experiment was conducted to investigate the effect of three earthworm species, representative of different ecological groups (epigeic: Lumbricus rubellus; endogeic: Aporrectodea caliginosa tuberculata; and anecic: Lumbricus terrestris), and their interactions on the bacterial community, and on nitrogen mineralization from ¹⁵N-labelled crop residue and from soil organic matter.Our results indicate that L. rubellus and L. terrestris enhanced mineralization of the applied crop residue whereas A. caliginosa had no effect. On the other hand, L. rubellus and A. caliginosa enhanced mineralization of the soil organic matter, whereas L. terrestris had no effect. The interactions between different earthworm species affected the bacterial community and the net mineralization of soil organic matter. The two-species interactions between L. rubellus and A. caliginosa, and L. rubellus and L. terrestris, resulted in reduced mineral N concentrations derived from soil organic matter, probably through increased immobilization in the bacterial biomass. In contrast, the interaction between A. caliginosa and L. terrestris resulted in increased bacterial growth rate and reduced total soil C. When all three species were combined, the interaction between A. caliginosa and L. terrestris was dominant. We conclude that the effects of earthworms on nitrogen mineralization depend on the ecological traits of the earthworm species present, and can be modified by species interactions. Knowledge of these effects can be made useful in the prevention of nutrient losses and increased soil fertility in agricultural systems, that typically have a low earthworm diversity.