溪流两边的湿地对其含氮量的贡献

本文对美国科罗拉多洛基山国家公园内ＬｏｃｈＶａｌｅ,小流域溪流商边的湿地土壤水溶液中的含氮量进行了研究,并比较了与其相邻的溪流中的含氮量。结果发现,溪流中的硝态氮含量显著高于３个湿地土壤水溶液中的,而氨态氮则并没有显著差异;溪流水中的ｐＨ值要显著高于土壤水溶液中的,而电导率又显著低于后者。同时,还发现取自不同地点的溪流水分的化学性质也有显著的不同,采自溪流支流水分的ｐＨ,电导率和硝态氮都要显著高于取自主溪流中的水分的。另外,还分析比较了３个湿地样地的地上部分生产力以及土壤和生物量中的碳和全氮含量。最后,我们认为溪流两边的湿地对溪流中的氮的含量并没有显著的影响。     

人工恢复黄河三角洲湿地土壤碳氮含量变化特征

为阐明湿地恢复对土壤碳氮含量的影响,本文以黄河三角洲芦苇湿地为研究对象,比较了退化区与连续淡水恢复区土壤pH值、盐分、有机碳、全氮、氨态氮、硝态氮的含量变化,研究结果表明,（1）随着恢复年限的增加,各样地各层土壤pH值总体上降低,电导率显著降低（P < 0.05）,表明退化湿地的人工恢复可明显降低其盐度;恢复区上层（0—20cm）土壤盐分均低于下层（20—40cm）,未恢复区则相反;（2）随着恢复年限的增加,0—20cm土壤有机碳、全氮含量增加,分别由恢复前的（7.710?0.756）g/kg、（0.66?0.021）g/kg增加到恢复7a后的（16.96?0.213）g/kg、（1.277?0.027）g/kg,恢复区有机碳、全氮含量在空间上的变化表现为上层大于下层;（3）各样地硝态氮含量均低于氨态氮含量,碳氮比值介于4—8之间;（4）相关分析表明,湿地恢复后土壤各因子间有显著的相关性,有机碳与全氮两者显著正相关,氨态氮与pH、盐分、有机碳、全氮均达到极显著正相关关系（P < 0.01）。该研究结果对于评估湿地恢复效应,指导湿地恢复的实践具有重要作用。     

水、肥添加对内蒙古典型草原土壤碳、氮、磷及pH的影响

采用野外田间实验和室内分析相结合的方法,研究了水分、氮肥和磷肥添加对内蒙古典型草原土壤碳、氮、磷含量及pH的影响。结果表明:土壤有机碳和全氮含量对水、肥添加无显著响应;水分添加降低了土壤铵态氮(31.1%,P<0.01)和硝态氮(7.9%,P<0.001)含量,而添加氮肥显著增加了土壤铵态氮和硝态氮的含量(P<0.001);添加磷肥显著增加了土壤全磷和速效磷含量,分别为不施磷肥的1.7和5.9倍(P<0.001);水分添加提高了土壤pH值,氮、磷肥的添加均不同程度地降低了土壤pH值(P<0.001)。表明:水分以及氮肥、磷肥的添加对内蒙古典型草原土壤的性质具有明显的调节作用,这将为退化草地的管理提供科学参考。     

高原鼠兔挖掘活动对土壤中氮素含量的影响

本文通过测定不同类型高原鼠兔鼠丘和鼠丘下0 ～ 10 cm 土壤中总氮、铵态氮和硝态氮的含量变化,分析了高原鼠兔挖掘活动对土壤中无机氮含量的影响,并通过测定高原鼠兔鼠丘密度,计算了每只高原鼠兔对氮素循环的贡献。研究结果表明:不同类型鼠丘土壤中总氮含量无明显变化,铵态氮、硝态氮和无机氮含量处理间变化趋势为当年鼠丘>两年鼠丘> 多年鼠丘> 对照。方差分析结果表明,硝态氮含量在5 月时差异显著,当年鼠丘和两年鼠丘显著大于多年鼠丘和对照,无机氮含量在5 月和9 月表现为当年鼠丘显著高于对照。在不同月份,铵态氮含量月间变化趋势为5 ～ 8 月逐渐降低,至9 月略有增加,硝态氮和无机氮含量呈现“高- 低- 高-低- 高”的“W”变化趋势。方差分析结果显示,铵态氮、硝态氮和无机氮含量月间变化显著。不同类型鼠丘下0 ～ 10 cm 土壤中铵态氮、硝态氮和无机氮含量处理间和月份间变化趋势与鼠丘土壤中变化趋势基本一致,但硝态氮和无机氮含量在当年鼠丘中均显著高于对照,且不同月份间铵态氮、硝态氮和无机氮的含量差异显著(P
< 0.05)。每只高原鼠兔挖掘活动所形成的鼠丘土壤中的铵态氮、硝态氮和无机氮分别增加了162.6 mg/ kg、355.1 mg/kg 和497.7 mg/ kg。     

青藏高原东缘窄叶鲜卑花灌丛生长季土壤无机氮对增温和植物去除的响应

为了揭示气候变暖背景下高寒灌丛土壤氮转化过程, 该文研究了青藏高原东缘窄叶鲜卑花(Sibiraea angustata)灌丛生长季节土壤硝态氮和铵态氮含量对增温和去除植物的响应。结果表明: 窄叶鲜卑花灌丛土壤硝态氮和铵态氮含量具有明显的季节动态。整个生长季节, 土壤硝态氮含量呈先增加后降低的趋势, 而铵态氮含量均表现为一直增加的趋势。在生长季初期和中期, 各处理土壤硝态氮含量均显著高于铵态氮含量, 而在生长季末期土壤硝态氮含量均显著低于铵态氮含量, 说明该区域土壤氮转化过程在生长季初期和中期以硝化作用为主, 而在生长季末期以氨化作用为主。不同时期土壤硝态氮和铵态氮含量对增温和去除植物的响应不同: 增温对硝态氮的影响主要发生在生长季中期和末期, 且因植物处理的不同而有显著差异, 增温仅在生长季中期使不去除植物样方铵态氮含量显著升高。去除植物对土壤硝态氮的影响仅表现在对照样方(不增温), 去除植物显著提高了生长季初期和中期土壤硝态氮含量, 显著降低了生长季末期土壤硝态氮含量; 同时去除植物显著降低了增温样方生长季中期土壤铵态氮含量。灌丛植被在生长季初期和中期可能主要吸收土壤硝态氮, 其吸收过程不受土壤增温的影响。     

Responses of soil inorganic nitrogen to increased temperature and plant removal during the growing season in a Sibiraea angustata scrub ecosystem of eastern Qinghai-Xizang Plateau

为了揭示气候变暖背景下高寒灌丛土壤氮转化过程, 该文研究了青藏高原东缘窄叶鲜卑花(Sibiraea angustata)灌丛生长季节土壤硝态氮和铵态氮含量对增温和去除植物的响应。结果表明: 窄叶鲜卑花灌丛土壤硝态氮和铵态氮含量具有明显的季节动态。整个生长季节, 土壤硝态氮含量呈先增加后降低的趋势, 而铵态氮含量均表现为一直增加的趋势。在生长季初期和中期, 各处理土壤硝态氮含量均显著高于铵态氮含量, 而在生长季末期土壤硝态氮含量均显著低于铵态氮含量, 说明该区域土壤氮转化过程在生长季初期和中期以硝化作用为主, 而在生长季末期以氨化作用为主。不同时期土壤硝态氮和铵态氮含量对增温和去除植物的响应不同: 增温对硝态氮的影响主要发生在生长季中期和末期, 且因植物处理的不同而有显著差异, 增温仅在生长季中期使不去除植物样方铵态氮含量显著升高。去除植物对土壤硝态氮的影响仅表现在对照样方(不增温), 去除植物显著提高了生长季初期和中期土壤硝态氮含量, 显著降低了生长季末期土壤硝态氮含量; 同时去除植物显著降低了增温样方生长季中期土壤铵态氮含量。灌丛植被在生长季初期和中期可能主要吸收土壤硝态氮, 其吸收过程不受土壤增温的影响。     

干旱荒漠区不同灌木根际与非根际土壤氮素的含量特征

选取广泛分布于阿拉善干旱荒漠区的白刺、霸王、红砂、沙冬青、沙木蓼、梭梭和驼绒藜7种不同的旱生灌木,研究其根际与非根际土壤各种形态氮素、有机碳的含量特征及土壤pH的变化.结果表明,相对于非根际土壤,根际土壤全氮、铵态氮、硝态氮分别平均高24.9%、24.5%和65.1%,土壤有机碳平均高出18.5%,土壤pH值平均低0.14个单位.根际与非根际土壤的全氮、铵态氮、硝态氮、有机碳和pH之间都呈现出了极显著差异(p<0.01).7种灌木根际土壤全氮、硝态氮和有机碳含量均比非根际土壤含量高.除沙冬青根际铵态氮含量较非根际低以外,其余6种灌木根际土壤铵态氮含量均高于非根际土壤.梭梭的根际土壤pH高于非根际,其它6种灌木均是根际pH低于非根际土壤.在根际与非根际,土壤有机碳与土壤全氮之间均呈显著相关,二者表现为线性关系.而土壤全氮与铵态氮在根际与非根际则均无相关性,全氮与硝态氮在根际和非根际土壤均显著相关,且二者也呈线性相关.     

不同土地利用类型下氮、磷在土壤剖面中的分布特征

在北京市东南郊大兴区采取了44处0～20cm,20～40cm,40～60cm,60～80cm,80～100cm5个不同深度的土壤剖面样品。按土地利用类型,采样点可分为农田、菜地、果园、林地、草地。土壤剖面中,由表层向深层,pH值升高,有机质、速效磷、全磷、硝态氮、全氮降低,且在20～40cm处有较大变化。表层土壤受土地利用影响,不同土地利用类型的土壤性质差别较大,尤以菜地土壤,pH为8.01低于其他类型土壤的平均值8.27,有机质、速效磷、全磷、硝态氮、全氮都高于其他类型的土壤,分别是其他类型土壤的110%～198%,355%～1629%,162%～224%,724%～1540%,130%～248%,速效磷和硝态氮远高于其他土壤。深层土壤性质差异不大,各项土壤性质差异随深度而变小,但菜地80～100cm处,硝态氮含量为18.8mgkg-1,是同深度其他类型土壤的175%～389%。土壤中硝态氮的积累情况,菜地>农田、果园、林地>草地。磷的积累与氮不同,速效磷在0～20cm大量积累,不同类型的土壤,速效磷积累差异显著,在40～60cm处,菜地速效磷含量是其他利用类型土壤的161%～602%;在80～100cm处,不同利用类型的土壤中速效磷无显著性差异。这一情况表明,菜地的过量施用氮、磷肥导致了土壤中的磷和氮大量积累,并以速效磷、硝态氮的形态向下淋溶并在深层土壤中积累。硝态氮在80～100cm的积累仍相当严重,有继续向下淋溶的可能,速效磷的淋溶在80～100cm处已较为微弱,其淋溶过程主要在0～60cm处。对速效磷和硝态氮的累积进行多元线性回归分析,发现速效磷与全磷含量有着良好的线性相关性,而与有机质和全氮含量关系不大。硝态氮则受土壤中pH、有机质和全氮3因素的共同影响。     

外来植物入侵对土壤氮有效性的影响

外来植物入侵对土壤氮循环和氮有效性的影响是入侵成功或进一步加剧的重要原因。通过对比相同研究地点入侵区域和无入侵区域的土壤原位氮状态差异, 探讨了外来植物入侵对土壤氮有效性的影响程度和生理生态学机制。基于107篇相关研究文献数据的整合, 发现植物入侵区域相对于无入侵区域土壤总氮、铵态氮、硝态氮、无机氮、微生物生物量氮含量显著增加, 增幅分别为(50 ± 14)%、(60 ± 24)%、(470 ± 115)%、(69 ± 25)%、(54 ± 20)%。土壤硝态氮含量增幅较大反映硝化作用增强, 这可能增加入侵植物硝态氮利用以及喜硝植物的共存。温带地区植物入侵后土壤的硝态氮含量增幅显著高于亚热带地区。固氮植物入侵后土壤的总氮和无机氮含量增幅均显著高于非固氮植物入侵。木本和常绿植物入侵后土壤的总氮含量增幅分别高于草本和落叶植物入侵; 而土壤铵态氮含量的增幅没有显著差异且与固氮入侵植物占比无明显关系; 然而硝态氮含量的增幅普遍较高且与固氮入侵植物占比显著正相关。外来入侵植物固氮功能以及凋落物质量和数量是影响土壤氮矿化和硝化过程的关键因素。该研究为理解外来植物入侵成功和加剧的机制以及入侵植物功能性状与土壤氮动态之间的关系提供了新的见解。     

外来植物入侵对土壤氮有效性的影响北大核心CSCD

外来植物入侵对土壤氮循环和氮有效性的影响是入侵成功或进一步加剧的重要原因。通过对比相同研究地点入侵区域和无入侵区域的土壤原位氮状态差异,探讨了外来植物入侵对土壤氮有效性的影响程度和生理生态学机制。基于107篇相关研究文献数据的整合,发现植物入侵区域相对于无入侵区域土壤总氮、铵态氮、硝态氮、无机氮、微生物生物量氮含量显著增加,增幅分别为(50±14)%、(60±24)%、(470±115)%、(69±25)%、(54±20)%。土壤硝态氮含量增幅较大反映硝化作用增强,这可能增加入侵植物硝态氮利用以及喜硝植物的共存。温带地区植物入侵后土壤的硝态氮含量增幅显著高于亚热带地区。固氮植物入侵后土壤的总氮和无机氮含量增幅均显著高于非固氮植物入侵。木本和常绿植物入侵后土壤的总氮含量增幅分别高于草本和落叶植物入侵;而土壤铵态氮含量的增幅没有显著差异且与固氮入侵植物占比无明显关系;然而硝态氮含量的增幅普遍较高且与固氮入侵植物占比显著正相关。外来入侵植物固氮功能以及凋落物质量和数量是影响土壤氮矿化和硝化过程的关键因素。该研究为理解外来植物入侵成功和加剧的机制以及入侵植物功能性状与土壤氮动态之间的关系提供了新的见解。     

Temporal coherence of two alpine lake basins of the Colorado Front Range, U.S.A.

1. Knowledge of synchrony in trends is important to determining regional responses of lakes to disturbances such as atmospheric deposition and climate change. We explored the temporal coherence of physical and chemical characteristics of two series of mostly alpine lakes in nearby basins of the Colorado Rocky Mountains. Using year‐to‐year variation over a 10‐year period, we asked whether lakes more similar in exposure to the atmosphere be‐haved more similarly than those with greater influence of catchment or in‐lake processes. 2. The Green Lakes Valley and Loch Vale Watershed are steeply incised basins with strong altitudinal gradients. There are glaciers at the heads of each catchment. The eight lakes studied are small, shallow and typically ice‐covered for more than half the year. Snowmelt is the dominant hydrological event each year, flushing about 70% of the annual discharge from each lake between April and mid‐July. The lakes do not thermally stratify during the period of open water. Data from these lakes included surface water temper‐ature, sulphate, nitrate, calcium, silica, bicarbonate alkalinity and conductivity. 3. Coherence was estimated by Pearson's correlation coefficient between lake pairs for each of the different variables. Despite close geographical proximity, there was not a strong direct signal from climatic or atmospheric conditions across all lakes in the study. Individual lake characteristics overwhelmed regional responses. Temporal coherence was higher for lakes within each basin than between basins and was highest for nearest neighbours. 4. Among the Green Lakes, conductivity, alkalinity and temperature were temporally coherent, suggesting that these lakes were sensitive to climate fluctuations. Water tem‐perature is indicative of air temperature, and conductivity and alkalinity concentrations are indicative of dilution from the amount of precipitation flushed through by snowmelt. 5. In Loch Vale, calcium, conductivity, nitrate, sulphate and alkalinity were temporally coherent, while silica and temperature were not. This suggests that external influences are attenuated by internal catchment and lake processes in Loch Vale lakes. Calcium and sulphate are primarily weathering products, but sulphate derives both from deposition and from mineral weathering. Different proportions of snowmelt versus groundwater in different years could influence summer lake concentrations. Nitrate is elevated in lake waters from atmospheric deposition, but the internal dynamics of nitrate and silica may be controlled by lake food webs. Temperature is attenuated by inconsistently different climates across altitude and glacial meltwaters. 6. It appears that, while the lakes in the two basins are topographically close, geologically and morphologically similar, and often connected by streams, only some attributes are temporally coherent. Catchment and in‐lake processes influenced temporal patterns, especially for temperature, alkalinity and silica. Montane lakes with high altitudinal gradients may be particularly prone to local controls compared to systems where coherence is more obvious.     

Climate-induced changes in high elevation stream nitrate dynamics

Mountain terrestrial and aquatic ecosystems are responsive to external drivers of change, especially climate change and atmospheric deposition of nitrogen (N). We explored the consequences of a temperature-warming trend on stream nitrate in an alpine and subalpine watershed in the Colorado Front Range that has long been the recipient of elevated atmospheric N deposition. Mean annual stream nitrate concentrations since 2000 are higher by 50% than an earlier monitoring period of 1991–1999. Mean annual N export increased by 28% from 2.03 kg N ha⁻¹ yr⁻¹ before 2000 to 2.84 kg N ha⁻¹ yr⁻¹ in Loch Vale watershed since 2000. The substantial increase in N export comes as a surprise, since mean wet atmospheric N deposition from 1991 to 2006 (3.06 kg N ha⁻¹ yr⁻¹) did not increase. There has been a period of below average precipitation from 2000 to 2006 and a steady increase in summer and fall temperatures of 0.12 °C yr⁻¹ in both seasons since 1991. Nitrate concentrations, as well as the weathering products calcium and sulfate, were higher for the period 2000–2006 in rock glacier meltwater at the top of the watershed above the influence of alpine and subalpine vegetation and soils. We conclude the observed recent N increases in Loch Vale are the result of warmer summer and fall mean temperatures that are melting ice in glaciers and rock glaciers. This, in turn, has exposed sediments from which N produced by nitrification can be flushed. We suggest a water quality threshold may have been crossed around 2000. The phenomenon observed in Loch Vale may be indicative of N release from ice features such as rock glaciers worldwide as mountain glaciers retreat.     

Impact of nutrient enrichment and alkalinization on periphyton communities in the New Jersey Pine Barrens

The impact of residential and agricultural development on stream periphyton communities in the New Jersey Pine Barrens was examined by comparison with communities in undeveloped areas. Watershed disturbance resulted in stream water primarily characterized by greatly elevated pH levels and nitrate concentrations. A total of 53 periphyton species were encountered in bimonthly samples over a one year period in the three disturbed and three undisturbed study streams. Species richness was significantly greater in the disturbed streams based on three criteria: the average number of species per stream on each sampling occasion (disturbed = 6.3; undisturbed = 4.9), the average number of species per stream for the entire year (disturbed = 19.3; undisturbed = 16.0), and the total number of species found in streams within a type (disturbed = 40; undisturbed = 31). Species composition also changed significantly as the result of disturbance. There appeared to be replacement of species characteristic of undisturbed Pine Barrens streams with species peripheral to the region. The expected effects of both elevated pH and nitrate were consistent with these results.     

Regional comparison of nitrogen export to Japanese forest streams

Nitrogen (N) emissions in Asian countries are predicted to increase over the next several decades. An understanding of the mechanisms that control temporal and spatial fluctuation of N export to forest streams is important not only to quantify critical loads of N, N saturation status, and soil acidification N dynamics and budgets in Japanese forested watersheds is not clear due to the lack of regional comparative studies on stream N chemistry. To address the lack of comparative studies, we measured inorganic N (nitrate and ammonium) concentrations from June 2000 to May 2001 in streams in 18 experimental forests located throughout the Japanese archipelago and belonging to the Japanese Union of University Forests. N concentrations in stream water during base flow and high flow periods were monitored, and N mineralization potential in soil was measured using batch incubation experiments. Higher nitrate concentrations in stream water were present in central Japan, an area that receives high rates of atmospheric N deposition. In northern Japan, snowmelt resulted in increased nitrate concentrations in stream water. The potential net N mineralization rate was higher in surface soil than in subsurface soil, and the high potential for N mineralization in the surface soil partly contributed to the increase in nitrate concentration in stream water during a storm event. Regional differences in the atmospheric N deposition and seasonality of precipitation and high discharge are principal controls on the concentrations and variations of nitrates in stream water in forested watersheds of Japan.     

Analysis of nitrogen saturation potential in Rocky Mountain tundra and forest: implications for aquatic systems

We employed grass and forest versions of the CENTURY model under a range of N deposition values (0.02–1.60 g N m^–2 y^–1) to explore the possibility that high observed lake and stream N was due to terrestrial N saturation of alpine tundra and subalpine forest in Loch Vale Watershed, Rocky Mountain National Park, Colorado. Model results suggest that N is limiting to subalpine forest productivity, but that excess leachate from alpine tundra is sufficient to account for the current observed stream N. Tundra leachate, combined with N leached from exposed rock surfaces, produce high N loads in aquatic ecosystems above treeline in the Colorado Front Range. A combination of terrestrial leaching, large N inputs from snowmelt, high watershed gradients, rapid hydrologic flushing and lake turnover times, and possibly other nutrient limitations of aquatic organisms constrain high elevation lakes and streams from assimilating even small increases in atmospheric N. CENTURY model simulations further suggest that, while increased N deposition will worsen the situation, nitrogen saturation is an ongoing phenomenon.     

Estimation of nitrate removal by riparian wetlands and streams in agricultural catchments: effect of discharge and stream order

1. Assessment of the role of landscape structures such as buffers is a necessary prerequisite for the sustainable management of water resources in an agricultural setting. 2. We monitored nitrate concentrations during interstorm periods at the outlet of 16 subcatchments of different orders within a catchment of 378 km². We characterised stream network, wetlands, agricultural practices and land cover and identified their relationships with nitrate fluxes and concentrations. 3. Two main factors controlled annual nitrate fluxes: the agricultural nitrogen surplus and the nature of the system comprising the wetland zone and adjoining watercourses. In the latter case, nitrate fluxes were reduced in proportion to the surface area of the riparian wetland and the flowpath distance of fluxes in the stream network. At the scale of the order‐6 stream, 53% of annual nitrate flux during interstorm periods was removed during transfer via the wetland and the river, corresponding to 21.1 kg N ha^?1 per year. 4. The influence of the riparian wetland zone/watercourse system increased during periods of low water level, explaining up to 64% of nitrate concentration variation among locations within the river network, but only 9% during periods of high water level. 5. The buffering role was stronger at higher stream orders, and the dependence on stream order was more apparent at low water level, when we observed mean nitrate concentrations in the order‐6 stream that were 47% lower than observed in order‐2 or order‐3 streams.     

Chemical Characteristics of Particulate, Colloidal, and Dissolved Organic Material in Loch Vale Watershed, Rocky Mountain National Park

The chemical relationships among particulate and colloidal organicmaterial and dissolved fulvic acid were examined in an alpine andsubalpine lake and two streams in Loch Vale Watershed, Rocky MountainNational Park. The alpine lake, Sky Pond, had the lowest dissolved organiccarbon (DOC) (0.37 mgC/L), the highest particulate carbon (POC) (0.13mgC/L), and high algal biomass. The watershed of Sky Pond is primarilytalus slope, and DOC and POC may be autochthonous. Both Andrews Creekand Icy Brook gain DOC as they flow through wet sedge meadows. Thesubalpine lake, The Loch, receives additional organic material from thesurrounding forest and had a higher DOC (0.66 mgC/L). Elemental analysis,stable carbon isotopic compositon, and ¹³C-NMR characterizationshowed that: 1) particulate material had relatively high inorganic contentsand was heterogeneous in compositon, 2) colloidal material was primarilycarbohydrate material with a low inorganic content at all sites; and 3)dissolved fulvic acid varied in compositon among sites. The lowconcentration and carbohydrate-rich character of the colloidal materialsuggests that this fraction is labile to microbial degradation and may beturning over more rapidly than particulate fractions or dissolved fulvic acid.Fulvic acid from Andrews Creek had the lowest N content and aromaticity,whereas Sky Pond fulvic acid had a higher N content and lower aromaticitythan fulvic acid from The Loch. The UV-visible spectra of the fulvic acidsdemonstrate that variation in characteristics with sources of organic carboncan explain to some extent the observed non-linear relationship betweenUV-B extinction coefficients and DOC concentrations in lakes.     

Hydrological and biological processes modulate carbon,nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec,Canada)

Changes in atmospheric deposition, stream water chemistry, and solute fluxes were assessed across 15 small forested catchments. Dramatic changes in atmospheric deposition have occurred over the last three decades, including a 70% reduction in sulphur (S) deposition. These changes in atmospheric inputs have been associated with expected changes in levels of acidity, sulphate and base cations in streams. Soil retention of S appeared to partially explain rates of chemical recovery. In addition to these changes in acid–base chemistry we also observed unexpected changes in nitrogen (N) biogeochemistry and nutrient stoichiometry of stream water, including decreased stream N concentrations. Among all catchments the average flux of dissolved inorganic nitrogen (DIN) was best predicted by average runoff, soil chemistry (forest floor C/N) and levels of acid deposition (both S and N). The rate of change in stream DIN flux, however, was much more closely correlated with reductions in rates of S deposition rather than those of DIN. Unlike DIN fluxes, the average concentrations as well as the rates of decline in streamwater nitrate (NO₃) concentration over time were tightly linked to stream dissolved organic carbon/dissolved organic nitrogen ratios DOC/DON and DON/TP rather than catchment characteristics. Declines in phosphorus adsorption with increasing soil pH appear to contribute to the relationship between C, N, and P in our study catchments. Our observations suggest that catchment P availability and its alteration due to environmental changes (e.g. acidification) might have profound effects on N cycling and catchment N retention that have been largely unrecognized.     

The effects of forest on stream water quality in two coastal plain watersheds of the Chesapeake Bay

Forest had varying effects on stream nutrients in two coastal plain basins of the Delmarva Peninsula, USA. In the Choptank basin, forest was strongly associated with low stream total nitrogen (TN) and nitrate (NO₃⁻) concentrations (r²0.70), and forest placement along first order streams was important in maintaining low stream nitrogen (N) concentrations (r²0.35). In addition, a multiple regression model explained 40% of the stream total phosphorus (TP) variance and indicated that forest directly adjacent to streams (0–100 m) acted as a TP source and forest further away (100–300 m) from streams acted as a TP sink. In contrast, stream nutrients in the nearby Chester basin demonstrated a strong relationship with soil hydrologic properties. Forest had no significant effect on stream N and P because the finer-textured soils, higher stream slopes, and higher runoff potential of the Chester basin appeared to result in less baseflow compared to that in the Choptank basin. This reduced the opportunity for forest to intercept N via plant uptake and denitrification in the high runoff potential soils of the Chester basin. The high percentage of stormflow (40%) coupled with high stream slopes resulted in high soil erosion potential, which may explain the higher TP stream concentrations measured in the Chester compared to that in the Choptank. Differences in the hydrologic pathway appear to explain the different effects of forest on water quality in these two basins.