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Kauri dieback, caused by Phytophthora agathidicida, is a biotic disturbance that poses a recent threat to the survival of kauri (Agathis australis) forests in New Zealand. Previous studies have shown that throughfall and stemflow play an important role in the kauri forests' internal nutrient cycle. However, the effects of P. agathidicida infection on canopy and forest floor nutrient fluxes in kauri forests remain unknown. Here, we measured throughfall, stemflow and forest floor water yield, nutrient (potassium, calcium, magnesium, manganese, silicon, sulfur, sodium, iron) concentrations and fluxes of ten kauri trees differing in soil P. agathidicida DNA concentration, and health status. We did not observe an effect of soil P. agathidicida DNA concentration on throughfall, stemflow, and forest floor water yield. Throughfall and forest floor nutrient concentrations and fluxes decreased (up to 50%) with increasing soil P. agathidicida DNA concentration. We found significant effects on potassium and manganese fluxes in throughfall; calcium and silicon fluxes in forest floor leachate. A decline in canopy and forest floor nutrient fluxes may result in soil nutrient imbalances, which in turn may negatively impact forest productivity and may increase the susceptibility of trees to future pathogen infection in these ecologically unique kauri forests. Given our findings and the increasing spread of Phytophthora species worldwide, research on the underlying physiological mechanisms linking dieback and plant–soil nutrient fluxes is critical. [ABSTRACT FROM AUTHOR]

As a cause of ecosystem disturbances, phytophagous insects are known to directly influence the element and organic matter (OM) cycling in ecosystems by their defoliation and excretion activity. This study focuses on the interplay between short‐term, insect herbivory, plant responses to feeding activity, rhizosphere processes, and belowground nutrient availability under nutrient‐poor soil conditions. To test the effects of insect herbivory on OM and nutrient cycling in an N‐limited pasture system, mesocosm laboratory experiments were conducted using Dactylis glomerata as common grass species and Chorthippus dorsatus, a widespread grasshopper species, to induce strong defoliating herbivory. 13CO2 pulse labeling was used together with labeled 15N feces to trace the fate of C in soil respiration at the beginning of herbivory, and of C and N in above‐ and belowground plant biomass, grasshopper, feces, bulk soil, soil microbial biomass, throughfall solutions, and soil solutions. Within five days, herbivory caused a reduction in aboveground grass biomass by about 34%. A linear mixed‐effects model revealed that herbivory significantly increased total dissolved C and N amounts in throughfall solutions by a factor of 4–10 (P < 0.05) compared with the control. In total, 27.6% of the initially applied feces 15N were translocated from the aboveground to the belowground system. A significant enrichment of 15N in roots led to the assumption that feces‐derived 15N was rapidly taken up to compensate for the frass‐related foliar N losses in light of N shortage. Soil microorganisms incorporated newly available 13C; however, the total amount of soil microbial biomass remained unaffected, while the exploitative grass species rapidly sequestered resources to facilitate its regrowth after herbivory attack. Heavy herbivory by insects infesting D. glomerata‐dominated, N‐deficient grasslands remarkably impacted belowground nutrient cycling by an instant amplification of available nutrients, which led to an intensified nutrient competition between plants and soil microorganisms. Consequently, these competitive plant–soil microbe interactions accelerated N cycling and effectively retained herbivory‐mediated C and N surplus release resulting in diminished N losses from the system. The study highlighted the overarching role of plant adaptations to in situ soil fertility in short‐term ecosystem disturbances. [ABSTRACT FROM AUTHOR]

Nature‐based solutions for water‐resource challenges require advances in the science of ecohydrology. Current understanding is limited by a shortage of observations and theories that can further our capability to synthesize complex processes across scales ranging from submillimetres to tens of kilometres. Recent developments in environmental sensing, data, and modelling have the potential to drive rapid improvements in ecohydrological understanding. After briefly reviewing advances in sensor technologies, this paper highlights how improved measurements and modelling can be applied to enhance understanding of the following ecohydrological examples: interception and canopy processes, root uptake and critical zone processes, and up‐scaled effects of land use on streamflow. Novel and improved sensors will enable new questions and experiments, while machine learning and empirical methods provide additional opportunities to advance science. The synergy resulting from the convergence of these parallel developments will provide new insight into ecohydrological processes and thereby help identify nature‐based solutions to address water‐resource challenges in the 21st century. [ABSTRACT FROM AUTHOR]

Even though the functioning of nutrient-poor forest ecosystems strongly depends on the cycling of various elements, rather little is known about the effects of fires on the fluxes of Al, Ca, Fe, K,Mg,Mn, Na, P and S. Solution fluxes at three different soil depths (organic (O) layer, upper mineral soil (A) and lower mineral soil (B) horizon) weremeasured every 2 weeks with free-draining lysimeters before and after low-intensity prescribed surface fires in a Scots pine forest in Germany. Measurements of element content in pre-fire litterfall and soilwere also conducted. Linear mixed-effectmodelling revealed that low-intensity fires caused a short-term (,3months) increase of element fluxes fromtheOlayer and a mediumterm (3-8 months) increase from the A horizon. This solute flush was followed by retention processes in the B horizon, except for S, Ca and Mg, which were removed from the soil system, probably because anion exchange sites favoured dissolved organic carbon over SO42-, and because Ca2+ and Mg2+ partially maintained the charge balance. Our findings indicated that fires affected nutrient-poor soil systems by causing a short-and medium-term element translocation from the O layer into the B horizon, which functioned as a retaining soil horizon by reducing the losses of important elements. [ABSTRACT FROM AUTHOR]

Agroforestry systems may play a critical role in reducing the vulnerability of farmers' livelihood to droughts as tree-based systems provide several mechanisms that can mitigate the impacts from extreme weather events. Here, we use a replicated throughfall reduction experiment to study the drought response of a cacao/ Gliricidia stand over a 13-month period. Soil water content was successfully reduced down to a soil depth of at least 2.5 m. Contrary to our expectations we measured only relatively small nonsignificant changes in cacao (−11%) and Gliricidia (−12%) sap flux densities, cacao leaf litterfall (+8%), Gliricidia leaf litterfall (−2%), soil carbon dioxide efflux (−14%), and cacao yield (−10%) during roof closure. However, cacao bean yield in roof plots was substantially lower (−45%) compared with control plots during the main harvest following the period when soil water content was lowest. This indicates that cacao bean yield was more sensitive to drought than other ecosystem functions. We found evidence in this agroforest that there is complementary use of soil water resources through vertical partitioning of water uptake between cacao and Gliricidia. This, in combination with acclimation may have helped cacao trees to cope with the induced drought. Cacao agroforests may thus play an important role as a drought-tolerant land use in those (sub-) tropical regions where the frequency and severity of droughts is projected to increase. [ABSTRACT FROM AUTHOR]

Abstract: Few studies deal with the origin and contribution of organic matter (OM) fluxes from forest canopies to the variability of DOM and nutrient dynamics in the forest floor. Recent studies indicate a microbial and phytophages origin of OM in the throughfall of temperate forest ecosystems and reported inputs of dissolved (&#60;0.45 μm) organic carbon (DOC) with throughfall range between 40 and 160 kg C ha−1 year−1. Data on particulate organic matter (0.45 μm&#60;POM&#60;2 mm) associated with throughfall fluxes are rare and not often considered in terrestrial ecosystem element budgets. Using field and laboratory experiments, we investigated the direct effects of canopy infestation by two functional groups of herbivores (sap and leaf feeders) on dissolved and particulate organic matter fluxes in throughfall and their impact on forest floor processes. Throughfall fluxes of particulate amino-nitrogen beneath infested spruce were about 60% and those of hexose-carbon beneath infested hardwood trees up to 70% higher compared to uninfested control trees. Corresponding fluxes to filtered samples showed no statistically significant differences. In column irrigation experiments, different levels of aphid infestation (uninfested, moderately, heavy infested) were simulated by adding honeydew to the irrigation solution. Forest floor solution chemistry from forest floor showed a significant reduction in NH4–N and NO3–N fluxes, slightly reduced DON fluxes and significantly increased CO2 effluxes following honeydew application. We suggest that the amount of POM compounds transported with throughfall solution and its subsequent effects on forest floor processes depend on the herbivore functional group and on the level of infestation. From both a quantitative and qualitative point of view, our data indicate that herbivory-mediated organic matter in particulate and dissolved forms contribute considerably to the overall throughfall input of organic substances into the forest floor. [Copyright &y& Elsevier]

Kauri dieback, caused by Phytophthora agathidicida , is an emergent threat to the ecologically unique and carbon-rich kauri (Agathis australis) forests in New Zealand. Our main aim was to assess the effect of kauri dieback on canopy and forest floor dissolved and particulate carbon (C) and nitrogen (N) fluxes. Throughfall and stemflow collectors and free-draining lysimeters were deployed underneath the canopy of ten kauri trees differing in their soil P. agathidicida DNA concentration and visual health status and sampled weekly to monthly over 1 y. Throughfall and forest floor dissolved C and N fluxes decreased significantly with increasing soil P. agathidicida DNA concentration which may be related to changes in leaf chemistry, leachable kauri leaf surface area and uptake of N by the understory vegetation. The observed alteration in dissolved and particulate C and N fluxes under P. agathidicida infected kauri trees could lead to long-term changes in biogeochemical processes (e.g. mineralization, nutrient availability) in these ecologically unique kauri forests. [ABSTRACT FROM AUTHOR]

The heterogeneous nature of the size and shape of particulate matter (PM) deposited onto forest canopies is acknowledged. However, it is uncertain how PM interacts with the forest canopy and how it is transported and cycled via the hydrological processes of throughfall and stemflow. To improve our understanding of particulate cycling in forested watersheds, this study quantifies the geometric configuration of PM in bulk precipitation, throughfall, stemflow, and upper soil (Oa horizon) solution in both leafed and leafless periods in a European beech (Fagus sylvatica L.) forest in Germany. Circular equivalence diameter, circularity, elongation ratio, perimeter-to-area ratio, and fractal dimension were calculated for all 43,278 individual particulates in bulk precipitation, throughfall, stemflow, and Oa solutions. Loss on ignition measurements were also conducted to determine the organic matter content of the particulates. From a physical point of view, the opposite trends for circular equivalence diameter and perimeter-to-area ratio of PM between stemflow or throughfall and bulk precipitation during both leafless and leafed periods were the most striking. For bulk precipitation, the PM's mean circular equivalence diameter was significantly larger in the leafless period than the leafed period, with the reverse observed for throughfall and stemflow. Mean perimeter-to-area ratios (μm−1) of PM of both stemflow and throughfall were significantly larger in the leafless period than the leafed period. The opposite trend was observed for bulk precipitation and Oa solution. The percent organic matter of PM was not statistically significantly different across solutions or canopy state. Our results indicate that the differential routing of PM through the canopy indeed influences the geometry of PM among solution types as compared to the bulk precipitation. The effects of these changes on the chemistry of the PM and its impact of particulate cycling, and the impacts of shifting seasonality with climate change, warrants further investigation. • Particulate matter (PM) transport through forests may be affected by its geometry. • Quantified PM geometry of bulk precipitation, throughfall, stemflow, Oa solution. • Differential routing of PM through the canopy and phenology influenced PM geometry. • Organic matter varied between 2.2% and 3.7% across PM sources and canopy state. • Examination of effects of differential PM geometry on elemental cycling is needed. [ABSTRACT FROM AUTHOR]

Dissolved and particulate organic carbon (DOC, POC) and nitrogen (DN, PN) are important constituents and indicators of the C and N dynamics in forested ecosystems, but little is known about fire effects on the fluxes of these elements. Biweekly fluxes at three different soil depths (organic layer O, mineral soil A, mineral soil B) were measured with zero-tension lysimeters before and after (prescribed) low-intensity surface fires in a Scots pine forest in central Germany. Measurements of soil organic C and total N concentrations, cold (soluble) and hot (labile) water-extractable DOC and DN, and soil respiration also were conducted for both pre- and post-burn bulk soils. Linear mixed-effect modelling (LMM) revealed that repeated low-intensity fire reduced DOC (− 64%) and DN (− 11%) fluxes in the organic layer, but increased soil CO 2 fluxes (+ 7%). A nutrient flush from the charred material into the A horizon, as indicated by an enhanced solution pH (+ 11%) and electrical conductivity (+ 68%), may have stimulated microbial activity, leading to enhanced DOC (+ 47%) and DN (+ 202%) production and fluxes, respectively. The B horizon was unaffected by the fire treatment and retained DOC and DN. In contrast to DOC and DN fluxes, POC and PN fluxes were less affected by the fire treatment and decoupled from those of dissolved organic matter (DOM). Our findings indicate that low-intensity surface fires can significantly affect generally nutrient-poor soil systems by causing a short-term flush (“hot moment”) of DOM in the mineral A horizon (vertical “hot spot”) and by sorption in the mineral B horizon. [ABSTRACT FROM AUTHOR]

Wildfires change the chemical composition of soil organic matter (SOM). Since the effects of fires on organic matter (OM) in soil solution are largely unknown, we sought to compare the quality of dissolved organic matter (DOM) and total organic matter (TOM = DOM + particulate OM) between burned and control sites. The sites were subjected to a low-intensity surface fire in a coniferous forest in Germany dominated by spodic Cambisols derived from Triassic sandstone. Soil solutions from three different soil horizons (O, Ah, Bw), and throughfall (TF) were analyzed using solid-state 13 C NMR spectroscopy, allowing us to track the initial fire impact on OM vertically through the soil profile and 70 days after the fire. In addition, organic layer samples were analyzed by 13 C NMR spectroscopy to compare the OM quality. Under control conditions, properties of SOM influence the chemical composition of DOM and TOM in soil solutions. However, with fire, there is an initial increase in aromatic C in SOM, but not in DOM and TOM. Seventy days after the fire treatment, the aromatic C fraction in soil solutions of O and Ah layers increased, possibly due to accelerated oxidation processes, which would make the aromatic C more water-soluble. Our findings highlight the importance of short-term low-intensity fire-induced changes on forest soils that are useful to those seeking to better understand and model the temporal variability in the response of soil chemistry to fire to improve our knowledge of TOM and DOM dynamics in soils. [ABSTRACT FROM AUTHOR]

Precipitation that has been intercepted by tree canopies and subsequently routed to the boles of trees is termed stemflow. Stemflow, because of its concentrated nature at the base of trees and the resultant depth equivalent inputs to near-stem soils, has been shown to contribute to overland flow and erosion, preferential and subsurface flow, perched water table development and groundwater recharge, and has been linked to by-pass flow promoting storm runoff in certain streams. Although the quantitative importance of stemflow has been captured at the tree-scale using the funneling ratio –a metric that compares the volume of stemflow generated by a tree to the volume of precipitation that would have fallen on an open area equivalent to the basal area of the tree-, representative expressions of the volumetric importance of stemflow at the stand-scale have been seldom used. As such, two metrics for expressing stemflow at the stand-scale are advocated for: the stand-scale funneling ratio and the stand-scale infiltration funneling ratio. The stand-scale funneling ratio is akin to the original funneling ratio, however, the stemflow volume from all trees within a stand is compared to the volume of water that would have fallen as rain in the open over an area equivalent to the basal area of that stand, while the stand-scale infiltration funneling ratio utilizes the total infiltration area of the stand in place of the stand basal area. Two case studies, one from a juvenile lodgepole pine stand in British Columbia, Canada, and one from a lowland tropical deciduous forest in Kratie, Cambodia, are used to illustrate the quantitative importance of stemflow using these two metrics even though stemflow is a relatively small percentage of rainfall in both forests. The stand-scale funneling ratio is also derived for eighteen mature forests from around the globe using the results from recent studies (January 2017 – June 2018, inclusive) and, when coupled with the stand-scale funneling ratios reported for 16 mature forests in the stemflow review paper of Levia and Germer (2015), it is shown that stemflow only needs to be as little as 0.5% of rainfall for stand-scale funneling ratios to exceed unity. As such, near-stem soils in mature forest environments typically receive inputs from stemflow that far exceed what they would have received from throughfall or precipitation alone. Typical values of the stand-scale infiltration funneling ratio that should be expected in mature forests given typical stemflow flow rates, stand basal areas, and surface soil saturated hydraulic conductivities in these environments is also discussed. The case for future studies to report these stand-scale funneling metrics is made so that this water input is expressed in a way that is commensurate with its ecohydrological importance and so that the role of stemflow in plant-soil interactions may be better conceptualized, permitting advances in critical zone science. [ABSTRACT FROM AUTHOR]