Phosphorus Export From Two Contrasting Rural Watersheds in the (Sub) Humid Ethiopian Highlands

Establishing worldwide sustainable and phosphorus efficient cropping systems is urgently needed because the supply of suitable phosphate rock is limited, and excess phosphorus in streams causes eutrophication. One of the impediments in the developing world for sustainable P practices is the lack of studies on P transport and its eventual disposition in the environment. One of these regions with few studies is the Ethiopian Highlands, with permeable volcanic soils. The objective was to establish baseline data on P watershed export in the (sub)humid highlands. Two contrasting watersheds were selected near Lake Tana. For 2 years, stream discharge and sediment, total P, dissolved P, and bioavailable particulate P concentrations were determined at the watershed outlet. The first watershed is the 57 km2 Dangishta, with lava intrusion dikes, forcing subsurface flow through faults to the surface and preventing gully formation. Subsurface flow was half of the 1745 mm annual precipitation, and surface runoff and erosion were minimal. The second watershed is the 9 km2 Robit Bata with 1,420 mm precipitation. The banks of several river banks were slumping. The upper part of the watershed generates saturation excess runoff. A hillslope aquifer in the lower part provided interflow. The average sediment concentrations of 10.5 g L−1 in the stream in Robit Bata (11 times that in Dangishta) reflected the sediments from banks slipping in the stream. The hydrology and the soil loss directly affected the phosphorus export. In Dangishta, the total P concentration averaged 0.5 mg L−1 at the outlet. In Robit Bata, the average total P concentration was 2 mg L−1. The bioavailable particulate P concentration was only twice the concentration in the runoff water. The low phosphorus content of the subsoil slipping in Robit Bata moderated biologically available particulate P at the outlet. Average dissolved P concentrations for both watersheds were around 0.1 mg L−1 in the low range found in temperate climates. It reflects the difference in length of time that phosphorus fertilizers have been applied. Our research concludes that commonly implemented practices such as strengthening river banks and stabilizing gully might not lead to improved water quality in Lake Tana.

Spatiotemporal evolution of ecosystem services in South China Karst and driving mechanism

To explore the above-mentioned problems, we analyzed the spatiotemporal evolution of ES (WY, SC, NPP, nitrogen and phosphorus export, HQ, CS, and Bio) in SCK, and combined policy (GFG), natural factors (MP, MT, PET, and VC) and human activities (PD, NL, GDP, FL, Forest, Grass, BL and UL) discussed the driving forces of ES changes in different landscapes. The results showed that from 1982 to 2018, WY, nitrogen Export, HQ, CS and Bio in SCK showed a decreasing trend, while SC and NPP showed an increasing trend. WY had a large decrease in all ES, a decrease of 13.23%; while NPP had a large increase, an increase of 19.16%. Comparing different landscape, the study found that SC, NPP, nitrogen and phosphorus export, CS and Bio in non-karst were higher than in karst, while HQ was the opposite. This study showed that natural factors (MP, MT, PET and VC) are the main factors of affecting ES in SCK, and found that the driving factors of ES indicators in different landscapes (karst and non-karst) are not the same. The study emphasized mountain enclosure for afforestation in karst suitable forests and barren hills to restore ES; fast-growing and pioneer species with a large amount of fine roots were planted in wasteland and rocky desertification areas to increase soil conservation services in the early ecosystems; through the policy of migration and employment, the human disturbance to nature can be reduced and habitat quality and biodiversity service can be improved.

Is phosphorus export from beech forest stands transport-limited of source-limited?

<p>Phosphorus (P) is one of the key limiting nutrients in forest ecosystem resulting in tight P-recycling strategies in natural forests. Hydrological fluxes in the subsurface during rainfall events can however lead to a relocation and export of P from the forest stands. We present results from six large-scale sprinkling experiments on three highly instrumented experimental hillslope in the Bavarian Forest, Black Forest, and the Swabian Alb in Germany that differ in their soil P stocks. We simulated an extreme 150 mm rainstorm with intensities between 12 and 15 mm/h. The aim of these experiments was to quantify the lateral and vertical fluxes of subsurface storm flow and phosphorus under a range of input fluxes and to identify differences in the degree of nutrient retention depending on the prevailing soil properties of the three forest sites.</p><p>We sprinkled the 200 m<sup>2</sup>, steep hillslopes with 60,000 l of isotopically (deuterium) labeled water for 11 h. Lateral subsurface flow was measured at three depths (10cm, 240cm, 300cm) at a 10 m wide trench at the bottom of the hillslope and with large zero-tension lysimeters (area of 0.6 m<sup>2</sup>) installed at four depths into the undisturbed soil profile. This setup allowed us to quantify the lateral and vertical fluxes of subsurface flow and phosphorus concentration during the experiment in 30 min temporal resolution. We found vertical subsurface flow to dominate over lateral flow by more than one order of magnitude. We could identify a P-flashing (i.e., high P concentrations) in the first 2 hours after start of subsurface flow across all soil depths. During the rest of the sprinkling the P-concentrations were lower but did not change significantly despite further increasing subsurface flow. We explored P concentrations as a function of subsurface flow and found for all observations, except those from the litter layer, to be chemostatic. We also found no change in P-concentrations with increasing new water fraction, calculated based on a two-component hydrograph separation approach using the deuterium label as tracer. However, when calculating the internal and total P-fluxes we realized that the majority of P, that was leached from the litter layer (i.e., 0.22 kg/ha at the P-poor site and 1.17 kg/ha at the P-rich site), was retained in the mineral soil. The total vertical and lateral losses from the experimental hillslope were small (i.e., 0.07 kg/ha at the P-poor site and 0.06 kg/ha at the P-rich site during each experiment).</p><p>Therefore, our results suggest that P-poor and P-rich forest ecosystems are efficiently retaining phosphors in their mineral soils. However, as phosphorus export is transport limited but not source limited an increase in the frequency of heavy rainstorms, as predicted under future climate conditions, might still lead to a relocation of phosphorus to soil depths below the depth of tree roots or even cause increased P-export from the forest stands.</p>