Intra and inter-annual climatic conditions have stronger effect than grazing intensity on root growth of permanent grasslands

Background and AimsUnderstanding how direct and indirect changes in climatic conditions, management, and species composition affect root production and root traits is of prime importance for grassland C sequestration service delivery.MethodsWe studied during two years the dynamics of root mass production with ingrowth-cores and annual above- and below-ground biomass (ANPP, BNPP) of upland fertile grasslands subjected for 10 years to a gradient of herbage utilization by grazing.ResultsWe observed strong seasonal root production across treatments in both a wet and a dry year but response to grazing intensity was hardly observed within growing seasons. In abandonment, spring and autumn peaks of root growth were delayed by about one month compared to cattle treatments, possibly due to later canopy green-up and lower soil temperature. BNPP was slightly lower in abandonment compared to cattle treatments only during the dry year, whereas this effect on ANPP was observed the wet year. In response to drought, the root-to-shoot biomass ratio declined in the abandonment but not in the cattle treatment, underlining higher resistance to drought of grazed grassland communities.ConclusionsRotational grazing pressure and climatic conditions variability had very limited effects on root growth seasonality although drought had stronger effects on BNPP than on ANPP.

Studying deep rooting and its value for crops

<p>Water and nutrients are distributed throughout the soil volume, and their ability to move towards the plant roots is highly restricted, in most cases to a few mm or less. This mean, that unlike the aboveground resources of light and CO<sub>2</sub> moving to the plants, roots need to grow towards the resources. Thus, for efficient resource uptake, roots need to be well distributed in the soil, both locally within the root zone and to grow deep to increase the overall volume of soil exploited. In crop production, deep rooting has been shown to be highly important for water and nitrogen use, and deep rooting is expected to contribute specifically to soil C sequestration.</p><p>Research into deep rooting and its functions is strongly restricted by the difficulties of studying roots hidden deeply in the soil. It is very laborious to access them, and even more difficult to set up experiments giving frequent and non-destructive measurements of the relevant parameters.  </p><p>In previous experiments, we have studied deep rooting of crops and cover crops and its effect on deep nitrogen uptake. By measuring roots and soil nitrogen to 2.5 m depth, we found that deep rooting was a main factor in nitrogen uptake and the reduction of nitrogen leaching loss. This showed how deep rooted species can be used to develop nitrogen efficient cropping systems. Further, it was shown that inclusion of deeper soil layers in the studies were critical for the conclusions to be drawn. Increasing the depth of study from e.g. 1 m to 2.5 m did not just moderate the conclusions and quantitative estimates, in several studies it basically changed conclusions that could be drawn.</p><p>Since 2015, we have built three new research platforms dedicated to detailed study of deep root growth and function. We have built a rhizobox facility consisting of 24 rhizoboxes each 4 m deep. The rhizoboxes are equipped with soil water sensors, and give access to observe the roots, take soil and root samples and inject tracers along the whole soil profile. In the field, we have built a platform, aimed at giving similar research possibilities, using long minirhizotrons, soil water sensors and metal access tubes for inserting ingrowth cores, all together giving valuable opportunities, though we cannot achieve the same easy access to the root zone as in the rhizoboxes. Finally, we have built a deep root phenotyping facility, using minirhizotrons to allow screening of 600 plant lines for root growth down to 3 meters depth, and allowing us to measure root activity of the lines by deep placement of isotope tracers.</p><p>These new facilities, and the research opportunities they give will be discussed, together with some of the first research results on deep roots we have obtained there.</p>

N : P stoichiometry and habitat effects on Mediterranean savanna seasonal root dynamics

Mediterranean grasslands are highly seasonal and co-limited by water and nutrients. In such systems, little is known about root dynamics which may depend on individual plant properties and environment as well as seasonal water shortages and site fertility. Patterns of root biomass and activity are affected by the presence of scattered trees, grazing, site management, and chronic nitrogen deposition, all of which can affect nutrient ratios and potentially cause development of nitrogen : phosphorus (N : P) imbalances in ecosystem stoichiometry. In this study we combined observations from minirhizotrons with root measurements from direct soil cores and ingrowth cores, along with measures of above-ground biomass to investigate seasonal root dynamics and root : shoot ratios in a Mediterranean tree–grass “savanna”. We investigated responses to soil fertility, using nutrient manipulation (N∕NP addition) and spatial microhabitat treatments between open-pasture and microhabitats under the tree canopy. Root dynamics over time were also compared with indices of above-ground growth drawn from proximal remote sensing. Results show distinct differences in root dynamics and biomass between treatments and microhabitats. Root biomass was higher with N additions, but did not differ from the control with NP additions in early spring. By the end of the growing season root biomass had increased with NP in open pastures but not higher than N added alone. In contrast, root length density (RLD) in pastures responded stronger to the NP than N-only addition, while beneath trees root biomass tended to be higher with only N. Even though root biomass increased, the root : shoot ratio decreased under nutrient treatments. Timing of root and shoot growth was reasonably well paired, although in autumn root growth appeared to be substantially slower than “regreening” of the system. We interpret these differences as a shift in community structure and/or root traits under changing stoichiometry induced by the fertilization. We also consider seasonal (phenology) differences in the strength and direction of effects observed.

Estimating Fine Root Production from Ingrowth Cores and Decomposed Roots in a Bornean Tropical Rainforest

Research highlights: Estimates of fine root production using ingrowth cores are strongly influenced by decomposed roots in the cores during the incubation period and should be accounted for when calculating fine root production (FRP). Background and Objectives: The ingrowth core method is often used to estimate fine root production; however, decomposed roots are often overlooked in estimates of FRP. Uncertainty remains on how long ingrowth cores should be installed and how FRP should be calculated in tropical forests. Here, we aimed to estimate FRP by taking decomposed fine roots into consideration. Specifically, we compared FRP estimates at different sampling intervals and using different calculation methods in a tropical rainforest in Borneo. Materials and Methods: Ingrowth cores were installed with root litter bags and collected after 3, 6, 12 and 24 months. FRP was estimated based on (1) the difference in biomass at different sampling times (differential method) and (2) sampled biomass at just one sampling time (simple method). Results: Using the differential method, FRP was estimated at 447.4 ± 67.4 g m−2 year−1 after 12 months, with decomposed fine roots accounting for 25% of FRP. Using the simple method, FRP was slightly higher than that in the differential method after 12 months (516.3 ± 45.0 g m−2 year−1). FRP estimates for both calculation methods using data obtained in the first half of the year were much higher than those using data after 12-months of installation, because of the rapid increase in fine root biomass and necromass after installation. Conclusions: Therefore, FRP estimates vary with the timing of sampling, calculation method and presence of decomposed roots. Overall, the ratio of net primary production (NPP) of fine roots to total NPP in this study was higher than that previously reported in the Neotropics, indicating high belowground carbon allocation in this forest.

Combined effects of altered N:P stoichometry and trees on Mediterranean savanna root dynamics

Mediterranean grasslands are highly seasonal and co-limited by water and nutrients. In such systems little is known about root dynamics which may depend on plant habit and environment as well seasonal water shortages and site fertility. This latter factor is affected by the presence of scattered trees and site management including grazing, as well as chronic nitrogen deposition, which may lead to N:P imbalance. In this study we combined observations from minirhizotrons collected in a Mediterranean tree-grass ecosystem (Spanish Dehesa), with root measurements from direct soil cores and ingrowth cores, and above-ground biomass to investigate seasonal root dynamics and root:shoot ratios. We investigated responses to soil fertility, using a nutrient manipulation (N / NP additions) and microhabitats effects between open pasture and under tree canopy locations. Root dynamics over time were compared with indices of above-ground growth drawn from proximal remote sensing (Normalised Difference Vegetation Index and Green Chromatic Coordinate derived from near-infrared enabled digital repeat photography). Results show distinct differences in root dynamics and biomass between treatments and microhabitats. Root biomass was higher with N additions, but not with NP additions in early spring, but by the end of the growing season root biomass had increased with NP in open pastures but not higher than N alone. In contrast, root length density (RLD) in pastures responded stronger to the NP than N only treatment, while beneath trees RLD tended to be higher with only N. Even though root biomass increased, root:shoot ratio decreased under nutrient treatments.We interpret these differences as a shift in community structure and/or root traits under changing stoichiometry and altered nutrient limitations. The timing of maximum root cover, as assessed by the minirhizotrons, did not match with above-ground phenology indicators at the site as root growth was low during autumn despite the greening up of the ecosystem. In other periods, roots responded quickly to rain events on the scale of days, matching changes in above-ground indices. Our results highlight the need for high resolution sampling to increase understanding of root dynamics in such systems, linkage with shifts in community structure and traits, and targeting of appropriate periods of the year for in-depth campaigns.

Responses of fine roots to experimental nitrogen addition in a tropical lower montane rain forest, Panama

Nitrogen (N) availability is a major control on fine-root growth and distribution with depth in forest soils. We investigated fine-root dynamics in response to N addition in a montane rain forest with N-limited above-ground production. Control and N-fertilized (125 kg urea-N ha−1 y−1) treatments were laid out in a paired-plot design with four replicates (each 40 × 40 m). During 1.5 y of treatment, fine root-biomass, necromass and production were assessed by sequential coring at three soil depths (organic layer, 0–10 cm and 10–20 cm mineral soil), whereas fine-root redistribution with depth was assessed by ingrowth cores. Total fine-root biomass, necromass and production in the controls were 458 ± 21 g m−2, 101 ± 9 g m−2 and 324 ± 33 g m−2 y−1, respectively. No significant difference at any depth was detected under N fertilization. Fine-root biomass in the organic layer decreased over time under N addition. At 10–20 cm in the mineral soil, fine-root biomass in ingrowth cores increased significantly after 1.5 y of N fertilization compared with the control. The increased available N may have induced the change in fine-root distribution to explore the deeper mineral soil for other nutrients which may cause additional limitation to above-ground production once N limitation is alleviated.

Fine root dynamics for forests on contrasting soils in the colombian Amazon

It has been hypothesized that in a gradient of increase of soil resources carbon allocated to belowground production (fine roots) decreases. To evaluate this hypothesis, we measured the mass and production of fine roots (<2 mm) by two methods: 1) ingrowth cores and, 2) sequential soil coring, during 2.2 years in two lowland forests with different soils in the colombian Amazon. Differences of soil resources were determined by the type and physical and chemical properties of soil: a forest on loamy soil (Ultisol) at the Amacayacu National Natural Park and, the other on white sands (Spodosol) at the Zafire Biological Station, located in the Forest Reservation of the Calderón River. We found that mass and production of fine roots was significantly different between soil depths (0–10 and 10–20 cm) and also between forests. White-sand forest allocated more carbon to fine roots than the clayey forest; the production in white-sand forest was twice (2.98 and 3.33 Mg C ha−1 year−1, method 1 and 2, respectively) as much as in clayey forest (1.51 and 1.36–1.03 Mg C ha−1 year−1, method 1 and 2, respectively); similarly, the average of fine root mass was higher in the white-sand forest (10.94 Mg C ha−1) than in the forest on clay soils (3.04–3.64 Mg C ha−1). The mass of fine roots also showed a temporal variation related to rainfall, such that production of fine roots decreased substantially in the dry period of the year 2005. Our results suggest that soil resources play an important role in patterns of carbon allocation in these forests; carbon allocated to above-and belowground organs is different between forest types, in such a way that a trade-off above/belowground seems to exist; as a result, it is probable that there are not differences in total net primary productivity between these two forests: does belowground offset lower aboveground production in poorer soils?

Assessment of Nutrient Limitation in Floodplain Forests with Two Different Techniques

We assessed nitrogen and phosphorus limitation in a floodplain forest in southern Georgia in USA using two commonly used methods: nitrogen to phosphorus (N:P) ratios in litterfall and fertilized ingrowth cores. We measured nitrogen (N) and phosphorus (P) concentrations in litterfall to determine N:P mass ratios. We also installed ingrowth cores within each site containing native soil amended with nitrogen (N), phosphorus (P), or nitrogen and phosphorus (N + P) fertilizers or without added fertilizer (C). Litter N:P ratios ranged from 16 to 22, suggesting P limitation. However, fertilized ingrowth cores indicated N limitation because fine-root length density was greater in cores fertilized with N or N + P than in those fertilized with P or without added fertilizer. We feel that these two methods of assessing nutrient limitation should be corroborated with fertilization trials prior to use on a wider basis.