The critical soil P levels for crop yield, soil fertility and environmental safety in different soil types

Background and aims

Sufficient soil phosphorus (P) is important for achieving optimal crop production, but excessive soil P levels may create a risk of P losses and associated eutrophication of surface waters. The aim of this study was to determine critical soil P levels for achieving optimal crop yields and minimal P losses in common soil types and dominant cropping systems in China.

Methods

Four long-term experiment sites were selected in China. The critical level of soil Olsen-P for crop yield was determined using the linear-plateau model. The relationships between the soil total P, Olsen-P and CaCl₂-P were evaluated using two-segment linear model to determine the soil P fertility rate and leaching change-point.

Results

The critical levels of soil Olsen-P for optimal crop yield ranged from 10.9 mg kg^?1 to 21.4 mg kg^?1, above which crop yield response less to the increasing of soil Olsen-P. The P leaching change-points of Olsen-P ranged from 39.9 mg kg^?1 to 90.2 mg kg^?1, above which soil CaCl₂-P greatly increasing with increasing soil Olsen-P. Similar change-point was found between soil total P and Olsen-P. Overall, the change-point ranged from 4.6 mg kg^?1 to 71.8 mg kg^?1 among all the four sites. These change-points were highly affected by crop specie, soil type, pH and soil organic matter content.

Conclusions

The three response curves could be used to access the soil Olsen-P status for crop yield, soil P fertility rate and soil P leaching risk for a sustainable soil P management in field.     

Methane emissions from wet grasslands on peat soil in a nature preserve

The area of wet grasslands on peat soil in the Netherlands is slowly increasing at the expense of drained, agriculturally used grasslands. This study aimed (i) to assess the contribution of wet grasslands on peat soil to methane (CH₄) emissions, and (ii) to explain differences among sites and between years in order to improve our understanding of controlling factors. For these purposes, a field study was conducted in the period 1994–1996 in the nature preserve Nieuwkoopse Plassen, which is a former peat mining and agricultural area. Net CH₄ emissions were measured weekly to monthly with vented closed flux chambers at three representative sites, and at ditches near these sites. Three-years average of CH₄ emissions was 7.9 g CH₄ m^–2 yr^–1 for Drie Berken Zudde, 13.3 for Koole, and 20.4 for Brampjesgat. Ditches near the sites emitted 4.2–22.5 g CH₄ m^–2 yr^–1. The time-course of CH₄ emissions for all experimental sites and years was fit with a multiple linear regression model with ground water level and soil temperature as independent variables. Lowering or raising the ground water level by 5 cm could decrease or increase CH₄ emissions by 30–50%. Therefore, ground water level management of these grasslands should be done with care.     

Assessment of nitrogen ceilings for Dutch agricultural soils to avoid adverse environmental impacts

In the Netherlands, high traffic density and intensive animal husbandry have led to high emissions of reactive nitrogen (N) into the environment. This leads to a series of environmental impacts, including: (1) nitrate (NO3) contamination of drinking water, (2) eutrophication of freshwater lakes, (3) acidification and biodiversity impacts on terrestrial ecosystems, (4) ozone and particle formation affecting human health, and (5) global climate change induced by emissions of N2O. Measures to control reactive N emissions were, up to now, directed towards those different environmental themes. Here we summarize the results of a study to analyse the agricultural N problem in the Netherlands in an integrated way, which means that all relevant aspects are taken into account simultaneously. A simple N balance model was developed, representing all crucial processes in the N chain, to calculate acceptable N inputs to the farm (so-called N ceiling) and to the soil surface (application in the field) by feed concentrates, organic manure, fertiliser, deposition, and N fixation. The N ceilings were calculated on the basis of critical limits for NO 3 concentrations in groundwater, N concentrations in surface water, and ammonia (NH3) emission targets related to the protection of biodiversity of natural areas. Results show that in most parts of the Netherlands, except the western and the northern part, the N ceilings are limited by NH 3 emissions, which are derived from critical N loads for nature areas, rather than limits for both ground- and surface water. On the national scale, the N ceiling ranges between 372 and 858 kton year(-1) depending on the choice of critical limits. The current N import is 848 kton year(-1). A decrease of nearly 60% is needed to reach the ceilings that are necessary to protect the environment against all adverse impacts of N pollution from agriculture.     

Modeled impacts of farming practices and structural agricultural changes on nitrogen fluxes in the Netherlands

In the Netherlands, nutrient emissions from intensive animal husbandry have contributed to decreased species diversity in (semi) natural terrestrial and aquatic ecosystems, pollution of groundwater, and possibly global warming due to N2O emissions. This paper presents the results of a modelling study presenting the impacts of both structural measures and improved farming practices on major nitrogen (N) fluxes, including NH3 and N2O emission, uptake, leaching, and runoff, in the Netherlands, using input data for the year 2000. Average annual fluxes (Gg N year(-1)) for the year 2000 were estimated at 132 for NH3 emission (160 Gg NH 3 year(-1)), 28 for N2O emission, 50 for N inflow to groundwater, and 15 for N inflow to surface water at a total N input of 1046. At this input, nitrate (NO3) concentrations in groundwater often exceeded the target of 50 mg NO3 l(-1), specifically in well-drained sandy soils. The ammonia (NH3) emissions exceeded emission targets that were set to protect the biodiversity of nonagricultural land. Improved farming practices were calculated to lead to a significant reduction in NH3 emissions to the atmosphere and N leaching and runoff to groundwater and surface water, but these improvements were not enough to reach all the targets set for those fluxes. Only strong structural measures clearly improved the situation. The NH3 emission target of 30 Gg NH3 year(-1), suggested for the year 2030, could not be attained, however, unless pig and poultry farming is completely banned in the Netherlands and all cattle stay almost permanently in low emission stables.     

‘Bioenergy from cattle manure? Implications of anaerobic digestion and subsequent pyrolysis for carbon and nitrogen dynamics in soil’

Cattle manure can be processed to produce bioenergy, resulting in by‐products with different physicochemical characteristics. To evaluate whether application of such bioenergy by‐products to soils would be beneficial compared with their unprocessed counterpart, we quantified differences in greenhouse gas emissions and carbon (C) and nitrogen (N) dynamics in soil. Three by‐products (¹⁵N‐labeled cattle manure, from which anaerobic digestate was obtained, which was subsequently pyrolysed) were applied to a loess and a sandy soil in a laboratory incubation study. The highest losses of soil C from biological activity (CO₂ respiration) were observed in manure treatments (39% and 32% for loess and sandy soil), followed by digestate (31% and and 18%), and biochar (15% and and 7%). Emissions of nitrous oxide (N₂O) ranged from 0.6% of applied N from biochar to 4.0% from manure. Isotope labeling indicated that manure N was most readily mineralized, contributing 50% to soil inorganic N. The anaerobic digestate was the only by‐product increasing the mineral N pool, while reducing emissions of N₂O compared with manure. In biochar treatments, less than 18.3% of soil mineral N derived from the biochar, while it did not constrain mineralization of native soil N. By‐products of anaerobic digestion and pyrolysis revealed soil fertility in addition to environmental benefits. However, the reported advantages lessen when the declining yields of C and N over the bioenergy chain are considered.     

Differentiating Intracellular from Extracellular Alkaline Phosphatase Activity in Soil by Sonication

Differentiating intracellular from extracellular enzyme activity is important in soil enzymology, but not easy. Here, we report on an adjusted sonication method for the separation of intracellular from extracellular phosphatase activity in soil. Under optimal sonication conditions [soil:water ratio  =  1/8 (w/v) and power density  =  15 watt ml^-1], the activity of alkaline phosphomonoesterase (phosphatase) in a Haplic Cambisol soil increased with sonication time in two distinct steps. A first plateau of enzyme activity was reached between 60 and 100 s, and a second higher plateau after 300 s. We also found that sonication for 100 s under optimal conditions activated most (about 80%) of the alkaline phosphatase that was added to an autoclaved soil, while total bacteria number was not affected. Sonication for 300 s reduced the total bacteria number by three orders of magnitude but had no further effects on enzyme activity. Our results indicate that the first plateau of alkaline phosphatase activity was derived from extracellular enzymes attached to soil particles, and the second plateau to the combination of extracellular and intracellular enzymes after cell lysis. We conclude that our adjusted sonication method may be an alternative to the currently used physiological and chloroform-fumigation methods for differentiating intracellular from extracellular phosphatase activity in soil. Further testing is needed to find out whether this holds for other soil types.     

Bronchoconstriction Induces TGF-β Release and Airway Remodelling in Guinea Pig Lung Slices

Airway remodelling, including smooth muscle remodelling, is a primary cause of airflow limitation in asthma. Recent evidence links bronchoconstriction to airway remodelling in asthma. The mechanisms involved are poorly understood. A possible player is the multifunctional cytokine TGF-β, which plays an important role in airway remodelling. Guinea pig lung slices were used as an in vitro model to investigate mechanisms involved in bronchoconstriction-induced airway remodelling. To address this aim, mechanical effects of bronchoconstricting stimuli on contractile protein expression and TGF-β release were investigated. Lung slices were viable for at least 48 h. Both methacholine and TGF-β₁ augmented the expression of contractile proteins (sm-α-actin, sm-myosin, calponin) after 48 h. Confocal fluorescence microscopy showed that increased sm-myosin expression was enhanced in the peripheral airways and the central airways. Mechanistic studies demonstrated that methacholine-induced bronchoconstriction mediated the release of biologically active TGF-β, which caused the increased contractile protein expression, as inhibition of actin polymerization (latrunculin A) or TGF-β receptor kinase (SB431542) prevented the methacholine effects, whereas other bronchoconstricting agents (histamine and KCl) mimicked the effects of methacholine. Collectively, bronchoconstriction promotes the release of TGF-β, which induces airway smooth muscle remodelling. This study shows that lung slices are a useful in vitro model to study mechanisms involved in airway remodelling.     

Seasonal variations in nitrous oxide losses from managed grasslands in The Netherlands

Seasonal and interannual variations in nitrous oxide (N₂O) losses from agricultural soils hamper the accurate quantification of the N₂O source strength of these soils. This study focuses on a quantification of seasonal and interannual variations in N₂O losses from managed grasslands. Special attention was paid to N₂O losses during the growing season and off-season as affected by grassland management. Fluxes of N₂O from grasslands with three different types of management and on four different soil types in the Netherlands were measured weekly during two consecutive years, using flux chambers. There were distinct seasonal patterns in N₂O losses, with large losses during spring, summer, and autumn but relatively small losses during the winter. These seasonal variations were related to fertilizer N application, grazing and weather conditions. Measurements of N₂O concentrations in soil profiles showed that a rise in groundwater level was accompanied by increased N₂O concentrations in the soil. Disregarding off-season losses would underestimate total annual losses by up to 20%, being largest for unfertilized grassland and smallest for N-fertilized grazed grassland. Total annual N₂O losses ranged from 0.5 to 12.9 kg N ha^-1 yr^-1 for unfertilized grasslands to 7.3 to 42.0 kg N ha^-1 yr^-1 for N-fertilized grazed grasslands. Despite the considerable interannual variations in N₂O losses, this study indicates that the results of measurements carried out in one year have predictive power for estimating N₂O losses in other years.     

Effects of grassland management on the emission of methane from intensively managed grasslands on peat soil

Methane (CH4) is the most important greenhouse gas next to CO2 and as such it contributes to the enhanced greenhouse effect. Peat soils are often considered as sources of CH4. Grasslands on the other hand are generally considered to be a net sink for atmospheric CH4. The aim of this study was twofold: (i) to quantify the net CH4 emission of intensively managed grasslands on peat soil in the Netherlands; and (ii) to assess the effects of grassland management, i.e. drainage, nitrogen (N) fertilization, and grazing versus mowing, on CH4 emission rates. Net CH4 emissions were measured weekly or biweekly for one year with vented closed flux chambers at two sites, one with a mean ground water level of 22 cm below surface and one with a mean ground water level of 42 cm. On each site there were three treatments: mowing without N application, mowing with N application, and grazing with N application. The dominating species was perennial ryegrass (Lolium perenne L.). Net CH4 emissions were low, in general in the range of -0.2 to 0.2 mg CH4 m-2 d-1. In the relatively warm summer of 1994, consumption of atmospheric CH4 peaked at 0.4 mg m-2 d-1. On an annual basis, the sites were net consumers of atmospheric CH4. However, the consumption was small: 0.31 to 0.08 kg CH4 ha-1 yr-1. Effect of mean ground water level was significant, but small. There were no significant effects of withholding N fertilization for some years and grazing versus mowing on net CH4 emissions. We conclude that grassland management of intensively managed grasslands on peat soil is not a suitable tool for reducing net CH4 emissions.     

Nitrogen loss from grassland on peat soils through nitrous oxide production

Nitrous oxide (N₂O) in soils is produced through nitrification and denitrification. The N₂O produced is considered as a nitrogen (N) loss because it will most likely escape from the soil to the atmosphere as N₂O or N₂. Aim of the study was to quantify N₂O production in grassland on peat soils in relation to N input and to determine the relative contribution of nitrification and denitrification to N₂O production. Measurements were carried out on a weekly basis in 2 grasslands on peat soil (Peat I and Peat II) for 2 years (1993 and 1994) using intact soil core incubations. In additional experiments distinction between N₂O from nitrification and denitrification was made by use of the gaseous nitrification inhibitor methyl fluoride (CH₃F).Nitrous oxide production over the 2 year period was on average 34 kg N ha^-1 yr^-1 for mown treatments that received no N fertiliser and 44 kg N ha^-1 yr^-1 for mown and N fertilised treatments. Grazing by dairy cattle on Peat I caused additional N₂O production to reach 81 kg N ha^-1 yr^-1. The sub soil (20–40 cm) contributed 25 to 40% of the total N₂O production in the 0–40 cm layer. The N₂O production:denitrification ratio was on average about 1 in the top soil and 2 in the sub soil indicating that N₂O production through nitrification was important. Experiments showed that when ratios were larger than l, nitrification was the major source of N₂O. In conclusion, N₂O production is a significant N loss mechanism in grassland on peat soil with nitrification as an important N₂O producing process.