Introduction of a natural resource balance indicator to assess soil organic carbon management: Agricultural Biomass Productivity Benefit.

The rising demand for feed and food has put an increasing pressure on agriculture, with agricultural intensification as a direct response. Notwithstanding the higher crop productivity, intensive agriculture management entails many adverse environmental impacts. Worldwide, soil organic carbon (SOC) decline is hereby considered as a main danger which affects soil fertility and productivity. The life cycle perspective helps to get a holistic overview when evaluating the environmental sustainability of agricultural systems, though the impact of farm management on soil quality aspects is often not integrated. In this paper, we introduce an indicator called Agricultural Biomass Productivity Benefit of SOC management (ABB_SOC), which, relying on natural resource consumption, enables to estimate the net effect of the efforts made to attain a better soil quality. Hereby the focus is put on SOC. First, we introduce a framework to describe the SOC trend due to farm management decisions. The extent to which remediation measures are required are used as a measure for the induced SOC losses. Next, ABB_SOC values are calculated as the balance between the natural resource consumption of the inputs (including remediation efforts) and the desired output of arable crop production systems. The models RothC and EU-Rotate_N are used to simulate the SOC evolution due to farm management and the response of the biomass productivity, respectively. The developed indicator is applied on several rotation systems in Flanders, comparing different remediation strategies. The indicator could be used as a base for a method to account for soil quality in life cycle analysis.

Regional analysis of groundwater phosphate concentrations under acidic sandy soils: Edaphic factors and water table strongly mediate the soil P-groundwater P relation.

Historic long-term P application to sandy soils in NW-Europe has resulted in abundant sorption, saturation and eventually leaching of P from soil to the groundwater. Although many studies recognize the control of site-specific factors like soil texture and phosphate saturation degree (PSD), the regional-scaled relevance of effects exerted by single factors controlling P leaching is unclear. Very large observational datasets of soil and groundwater P content are furthermore required to reveal indirect controls of soil traits through mediating soil variables. We explored co-variation of phreatic groundwater orthophosphate (o-P) concentration and soil factors in sandy soils in Flanders, Belgium. Correlation analyses were complemented with an exploratory model derived using 'path analysis'. Data of oxalate-extractable Al, Fe, P and pHKCl, phosphate sorption capacity (PSC) and PSD in three depth layers (0-30, 30-60, 60-90 cm), topsoil SOC, % clay and groundwater depth (fluctuation) were interpolated to predict soil properties on exact locations of a very extensive net of groundwater monitoring wells. The mean PSD was only poorly correlated to groundwater o-P concentration, indicating the overriding control of other factors in the transport of P to the groundwater. A significant (P < 0.01) positive non-linear relationship was found between groundwater o-P concentrations and pHKCl for all depth layers. Likewise, lower SOC% (P < 0.01) and shallower groundwater level (MHL or MLL) corresponded (P < 0.01) with higher o-P concentrations. Groundwater o-P unexpectedly correlated positively to clay% and path analysis indicated this to be an indirect effect of the groundwater level. Path analysis furthermore indicated an important indirect control of pH on groundwater o-P concentrations and a considerable direct effect of Pox, 0-90, Alox, 0-90 and MHL. The fact that groundwater o-P concentration was stronger controlled by soil pH and groundwater table depth than by PSD indicates the likely oversimplification of the latter index to measure the long-term potential risk of P leaching. Accounting for controls on leaching not included in PSD via an alternative index, however, seems problematic as in Flanders for example groundwater o-P turned out to be higher in finer textured soils or soils with higher pedogenic Fe content, probably because of their lower pedogenic Al content and higher soil pH. Path analysis of extensive soil and groundwater datasets seems a viable way to identify prime local determinants of soil P leaching and could be further on used for 'ground-truthing' more complex P-migration simulation models.

Field-scale assessment of direct and indirect effects of soil texture on organic matter mineralization during a dry summer.

Soil texture plays a crucial role in organic matter (OM) mineralization through both direct interactions with minerals and indirect effects on soil moisture. Separating these effects could enhance the modelling of soil organic carbon (SOC) dynamics under climate change scenarios. However, the attempts have been limited small-scale experiments. Here, we studied the effects of soil texture on added OM mineralization in loamy sand, loam and silt loam soils in nine agricultural fields in Flanders, Belgium. Soil moisture, temperature, groundwater table depth and the mineralization of 13C-labeled ryegrass were monitored in buried mesocosms for approximately three months during a dry summer. Ryegrass-C mineralization was lowest in the loamy sand (39 ± 7 %) followed by silt loam (48 ± 7 %) and loam (63 ± 5 %) soils, challenging the current clay%-based moderation of C-mineralization rates in soil models. Soil temperature was not influenced by soil texture, whereas soil moisture was indeed dependent on soil texture. It appears that capillarity sustained upward water supply from groundwater to the topsoil in loam and silt loam soils but not in loamy sand soil, although this difference in capillary rise could not fully explain the higher moisture content in loam than that in silt loam soils. Additionally, soil texture only impacted remnant added ryegrass pieces (>500 µm) but not the finer ryegrass-derived SOC (<500 µm), which might point at the important indirect control of texture on OM mineralization during prolonged summer drought. However, these effects are only manifested during drought when no other factors (e.g., groundwater depth or subsurface water flows) exert an overriding impact on the soil water balance. Overall, our findings highlight the need to properly incorporate the indirect effects of soil texture on OM mineralization into soil carbon models to accurately predict soil C stocks under future climate change scenarios.

Chemical staining of particulate organic matter for improved contrast in soil X-ray µCT images.

Degradability of organic matter (OM) in soil depends on its spatial location in the soil matrix. A recent breakthrough in 3D-localization of OM combined dual-energy X-ray CT-scanning with OsO4 staining of OM. The necessity for synchrotron-based µCT and the use of highly toxic OsO4 severely limit applications in soil biological experiments. Here, we evaluated the potential of alternative staining agents (silver nitrate, phosphomolybdenic acid (PMA), lead nitrate, lead acetate) to selectively enhance X-ray attenuation and contrast of OM in CT volumes of soils containing specific mineral soil particle fractions, obtained via lab-based X-ray µCT. In comparison with OsO4, administration of Ag+ and Pb2+ resulted in insufficient contrast enhancement of OM versus fine silt (< 20 µm) or clay (< 2 µm) mineral particles. The perfusion procedure used in this work induced changes in soil structure. In contrast, PMA staining resulted in a selective increase of OM's attenuation contrast, which was comparable to OsO4. However, OM discrimination from other soil phases remained a challenge. Further development of segmentation algorithms accounting for grey value patterns and shape of stained particulate OM may enable its automated identification. If successful in undisturbed soils, PMA staining may form an alternative to OsO4 in non-synchrotron based POM detection.

Nitrogen Limitations on Microbial Degradation of Plant Substrates Are Controlled by Soil Structure and Moisture Content.

Mineral nitrogen (N) availability to heterotrophic micro-organisms is known to impact organic matter (OM) decomposition. Different pathways determining the N accessibility depend to a large extent on soil structure. Contact between soil mineral and OM substrate particles can facilitate N transport toward decomposition hot spots. However, the impact of soil structure on N availability to microbes and thus heterotrophic microbial activity and community structure is not yet fully understood. We hypothesized that carbon mineralization (Cmin) from low-N substrate would be stimulated by increased N availability caused by closer contact with soil particles or by a higher moisture level, enhancing potential for N-diffusion. Under opposite conditions retarded heterotrophic activity and a dominance of fungi were expected. A 128-days incubation experiment with CO2 emission monitoring from artificially reconstructed miniature soil cores with contrasting soil structures, viz. high or low degree of contact between soil particles, was conducted to study impacts on heterotrophic activity. The soil cores were subjected to different water filled pore space percentages (25 or 50% WFPS) and amended with either easily degradable OM high in N (grass) or more resistant OM low in N (sawdust). X-ray µCT image processing allowed to quantify the pore space in 350 µm around OM substrates, i.e., the microbial habitat of involved decomposers. A lower local porosity surrounding sawdust particles in soils with stonger contact was confirmed, at least at 25% WFPS. Mineral N addition to sawdust amended soils with small particle contact at 25% WFPS resulted in a stimulated respiration. Cmin in the latter soils was lower than in case of high particle contact. This was not observed for grass substrate particles or at 50% WFPS. The interactive effect of substrate type and soil structure suggests that the latter controls Cmin through mediation of N diffusion and in turn N availability. Phospholipid fatty acid did not reveal promotion of fungal over bacterial biomarkers in treatments with N-limited substrate decomposition. Combining X-ray µCT with tailoring soil structure allows for more reliable investigation of effects on the soil microbial community, because as also found here, the established soil pore network structure can strongly deviate from the intended one.

Increasing trends of dissolved organic nitrogen (DON) in temperate forests under recovery from acidification in Flanders, Belgium.

We evaluated trends (2005-2013) and patterns of dissolved organic nitrogen (DON) and its ratio with dissolved organic carbon (DOC), DOC:DON in atmospheric deposition and soil solution of five Level II plots of the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) in Flanders, Northern Belgium. The primary aim was to confirm positive postulated trends in DON levels and DOC:DON under on-going recovery from acidification. The DON concentrations (0.95-1.41 mg L(-1)) and fluxes (5.6-8.3 kg ha(-1)y(-1)) in throughfall were about twice as high compared to precipitation in the open field (0.40-0.48 mg L(-1), 3.0-3.9 kg ha(-1)y(-1)). Annual soil profile leaching losses of DON varied between 1.2 and 3.7 kg ha(-1)y(-1). The highest soil DON concentrations and fluxes were observed beneath the O horizon (1.84-2.36 mg L(-1), 10.1-12.3 kg ha(-1)y(-1)). Soil solution concentrations and fluxes of DON showed significant increasing trends. Temporarily soil solution DOC:DON rose following an exceptionally long spring drought in 2007, suggesting an effect of drying and rewetting on DOM composition. Further research is needed to test the dependence of DON and DOC:DON on factors such as latitude, forest cover, length of the growing season, hydrology and topography. Nonetheless, even with considerable variation in soil type, level of base saturation, and soil texture in the five included ICP Forests Level II plots, all data revealed a proportionally larger positive response of DON flux than DOC to recovery from acidification.

Quantifying the Contribution of Entire Free-Living Nematode Communities to Carbon Mineralization under Contrasting C and N Availability.

To understand the roles of nematodes in organic matter (OM) decomposition, experimental setups should include the entire nematode community, the native soil microflora, and their food sources. Yet, published studies are often based on either simplified experimental setups, using only a few selected species of nematode and their respective prey, despite the multitude of species present in natural soil, or on indirect estimation of the mineralization process using O2 consumption and the fresh weight of nematodes. We set up a six-month incubation experiment to quantify the contribution of the entire free living nematode community to carbon (C) mineralization under realistic conditions. The following treatments were compared with and without grass-clover amendment: defaunated soil reinoculated with the entire free living nematode communities (+Nem) and defaunated soil that was not reinoculated (-Nem). We also included untreated fresh soil as a control (CTR). Nematode abundances and diversity in +Nem was comparable to the CTR showing the success of the reinoculation. No significant differences in C mineralization were found between +Nem and -Nem treatments of the amended and unamended samples at the end of incubation. Other related parameters such as microbial biomass C and enzymatic activities did not show significant differences between +Nem and -Nem treatments in both amended and unamended samples. These findings show that the collective contribution of the entire nematode community to C mineralization is small. Previous reports in literature based on simplified experimental setups and indirect estimations are contrasting with the findings of the current study and further investigations are needed to elucidate the extent and the mechanisms of nematode involvement in C mineralization.