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Modelling reduced coastal eutrophication with increased crop yields in Chinese agriculture.

Introduction

China is experiencing rapid economic, social and cultural development. Chinese agriculture has been transforming from a low-yielding system to a system with high nutrient inputs and high crop yields. Increasing yields was needed to meet the food demand of a growing Chinese population. Today, nutrient inputs to agriculture are generally high in China (Ma et al. 2012). Meanwhile, nitrogen (N) and phosphorus (P) use efficiencies in the food production system are generally low. Chinese farmers often over-fertilise crops (Ma et al. 2008), and animal manure is not always recycled on the land as fertiliser.

As a result of low nutrient use efficiencies, losses of N and P from food production to the environment are high in China. Herein we focus on the flow of N and P from land to rivers and coastal seas. Agriculture is an important source of anthropogenic N and P inputs to coastal seas (Yang et al. 2007; Yin et al. 2007). River export of nutrients has been increasing rapidly over recent decades (Qu and Kroeze 2012; Liu et al. 2013); for example, between 1970 and 2000, river export of dissolved inorganic N and P (DIN and DIP respectively) increased between three and sevenfold and the dominant source of DIN was synthetic fertiliser (Qu and Kroeze 2012). Individual rivers, such as the Chang Jiang River, show a similar trend, with nutrient export in 1997 being 12-fold higher than that in 1968 (Yan et al. 2003; Le et al. 2010).

Nutrient enrichment in coastal waters may lead to eutrophication and increases the risk of harmful algal blooms (Gu et al. 2008; Liu et al. 2009; Qu and Kroeze 2010,2012). The associated damage to the marine environment can be irreversible and may bring about massive economic loss. In 2014, 56 red tides were observed in Chinese coastal areas, and green tides affected more than 5000 km2 of the coastal waters in the Yellow Sea (State Oceanic Administration (SOA) 2015). In the future, coastal eutrophication may further increase in China (Strokal et al. 2016).

Clearly, there is a need to produce more food with less environmental pollution in China. This would imply a change from the current high-input, high-output agricultural systems to a sustainable system with high nutrient use efficiency, high crop yields and reduced N and P losses to the environment (Chen et al. 2014). Recent studies (e.g. Chen et al. 2006, 2011 ; Fan et al. 2009; Cui et al. 2010; Zhang et al. 2011) have claimed that 'Double High Agriculture' (DAH) is a promising way to reduce nutrients losses to the environment while securing food demand. DAH is an agricultural practice that aims to achieve high crop yields and high nutrient use efficiencies in crop production. The practice includes using available nutrient management strategies and technologies to achieve the aim in different agricultural systems. A study of the DAH practice conducted in the North China Plain and Taihu region with 49 field experiments showed that fertiliser use in the field was reduced by 30-60% from 2003 to 2006 while crop yields were maintained (Ju et al. 2009). Such reductions in fertiliser use are likely to reduce nutrient losses to rivers, and thus to coastal seas. However, quantitative analyses of the effects of high crop yield and high nutrient use efficiency on coastal eutrophication are lacking.

Thus, the aim of the present study was to analyse the effect of increasing crop yields on future coastal eutrophication. To this end, we analysed scenarios using the Global Nutrient Export from Watersheds 2 (NEWS 2) model (https://marine.rutgers. edu/globalnews/), using the Global Orchestration (GO) scenario for 2050, from the Millennium Ecosystem Assessment (Alcamo et al. 2005; Seitzinger et al. 2010), as a reference. Our alternative scenarios assumed that nutrient use efficiencies and crop yields would increase simultaneously. Using the NEWS 2 model, we analysed the effects of improved nutrient management on N and P export by rivers in China in 2050 and the associated coastal eutrophication.

The present study focuses on the scale of river basins. In our scenarios, we explored how different farming practices affected coastal eutrophication. The findings of the present study may lead to future studies that link nutrient flows at a farm level to nutrient flows to rivers. Nutrient inputs to land and losses from the agricultural system depend on agricultural practices at the farm level. Nutrient recycling often occurs within or between farms. More detailed future analyses could consider variations between and within farms. The scenarios described herein are a starting point and may provide a basis for assessments of the environmental effect of specific farming practices, such as manure recycling (MR), inter-cropping or alternative irrigation methods.

Materials and methods

Study area

Sixteen river basins were selected from Global NEWS 2 (Fig. 1 ; Mayorga et al. 2010; Qu and Kroezc 2010). These rivers drain into three major Chinese seas; the Huang He, Liao, Haiho/ Yongding, Luan, Dalin He and Xiaoqing He rivers drain into the Bohai Gulf; the Chang Jiang, Huai, Fuchun Jiang, Menjiang, Yalu and Qujiang rivers discharge into the Yellow Sea; and the Zhujiang, Dongjiang, Hanjiang and Jiulong rivers drain into the South China Sea.

Global NEWS 2 model

The Global NEWS 2 model quantifies annual river export of N, P, silica (Si) and carbon (C) in dissolved inorganic (DIN, DIP, DSi), dissolved organic (DON, DOP, DOC) and particulate (PN, PP) forms in a spatially explicit way and as a function of human activities on land (e.g. crop and animal production, sewage discharge and water consumption; see below). The river export of nutrients is quantified in yields (kg [km.sup.-2] [year.sup.-1]) and loads (Mg [year.sup.-1]) for past (1970 and 2000) and future (2030 and 2050) years for over 6000 rivers worldwide (Mayorga et al. 2010; Seitzinger et al. 2010). Dissolved forms of nutrients are calculated based on the mass balance approach (Mayorga et al. 2010). Particulate nutrient forms are quantified based on a linear regression analysis, taking into account suspended solids ('Quantifying particulate forms of nutrient and dissolved silica in Global NEWS 2' in the supplementary material; Beusen et al. 2005; Mayorga et al. 2010).

River networks of the Global NEWS 2 model were defined based on the Stimulated Topological Network (STN-30p) river system with a global 0.5[degrees] x 0.5[degrees] grid cell. Hydrology was quantified by the Water Balance Model Plus (WB[M.sub.plus]; Fekete et al. 2010). Land use, ecosystems and nutrient inputs to watersheds from point and diffuse sources were taken from the Integrated Model to Assess the Global Environment (IMAGE) model (Bouwman et al. 2009; Van Drecht et al. 2009).

Here we briefly introduce the calculations for the dissolved forms of nutrients. Equations to quantify river export of dissolved nutrients are summarised in the Fig. S1. River export of dissolved forms of N and P results from point and diffuse sources, corrected for nutrient losses and retention in the soil and rivers (e.g. denitrification, and water removal for human consumption). Point sources include sewage effluent, discharged directly to rivers or after wastewater treatment. Diffuse sources include nutrient flows from natural and anthropogenic sources such as biological N2 fixation, atmospheric N deposition, P weathering from non-agricultural areas, application of synthetic fertilisers and animal manure and the leaching of organic matter from soils (Mayorga et al. 2010).

In the Global NEWS 2 model, land use types are classified into wetland rice, upland crop, legume and grassland (Mayorga et al. 2010). In the present study, the International Assessment of Agricultural Knowledge, Science, and Technology for Development (IAASTD; Bouwman et al. 2013) was used to estimate fertiliser input to different land use types in the Global NEWS 2 model river basins in 2050 ('Transferring data from IAASTD to Global NEWS-2' and Box S1, available as supplementary material to this paper).

Mayorga et al. (2010) describe the Global NEWS 2 model in detail. Seitzinger et al. (2010) elaborate on the implementation of the Millennium Ecosystem Assessment (MA) scenarios into the model. Bouwman et al. (2009) and Van Drecht et al. (2009) provide more information on how diffuse and point sources of nutrients in rivers are quantified by the model. These studies also provide the sources of model inputs. Fekete et al. (2010) provide information of model inputs for hydrology and climate.

The Global NEWS 2 model was calibrated and validated for large world basins, including Chinese rivers such as the Huang He and Chang Jiang (Mayorga et al. 2010). For Chinese rivers, model performance was evaluated by Qu and Kroeze (2012) and Strokal et al. (2014). According to their results, model performance is good for DIN and DIP. For example, according to Pearson's coefficient of determination (R2), the Global NEWS 2 model can explain 96% of the variance in the measured DIN and DIP yields for 16 selected Chinese rivers (Strokal et al. 2014). Sensitivity analysis revealed that the model is sensitive to parameters related to nutrient retention. A so-called nutrient export fraction is used to account for nutrient retention in soils, rivers and sediments. The model accounts for sedimentation in rivers and the effects of dams. In addition, human water consumption is accounted for.

Scenario descriptions

In the present study, we analysed five scenarios for 2050: (1) GO; (2) improved practice (IP); (3) an integrated soil-crop system management (1SSM); (4) IP with advanced manure management, namely MR (IP-MR); and (5) ISSM with advanced manure management (ISSM-MR). GO is the reference scenario and the other four scenarios assumed implementation of the high nutrient use efficiency and high crop yield agricultural systems (i.e. DHA scenarios). In all scenarios, we assumed that there were no direct (point source) inputs of manure to rivers.

GO was taken from the MA; it assumes a globalising world and a society with a reactive attitude towards environmental problems. Details on the MA are provided in 'Scenarios descriptions (extended)' (Supplementary material), as well as in other publications (e.g. Alcamo et al. 2005; Seitzinger et al. 2010). GO assumes that food production for a growing Chinese population leads to an increase in synthetic fertiliser use by a factor between 1.4 for N and 3 for P from 2000 to 2050 (Fig. 2a, b). Moreover, water demand for crop production was assumed to increase and sewage systems were assumed to improve in the future, although there would still be moderate N and P removal rates in sewage treatment (Seitzinger et al. 2010).

The four DHA scenarios were developed on the basis of a series of studies from the Science and Technology Backyards (STBs) platform in China (Chen et al. 2006, 2014; Zhang et al. 2016). This platform involves agronomists and local famers using participatory approaches to transfer technologies and knowledge. By 2015, there were 72 STBs established in 21 provinces, with these STBs covering 22 different agricultural systems in China (Zhang et al. 2016). The main purpose of STBs is to transfer knowledge from scientists to farmers and to provide region- or farm-specific recommendations for crop production. Crop production increased rapidly over the past 20 years. For example, the maize yield increased from 3.9 to 5.9 Mg ha-1 in the intensive maize-growing provinces of the North China Plain from the 1980s to the 2010s. Policies regarding synthetic fertiliser subsidies played an important role in this increase (Fischer et al. 2014). To further increase crop yields, region-or farm-specific recommendations on resource use are needed. In the STBs program, 153 field experiments were performed to analyse and improve nutrient use efficiencies and crop yields that can be achieved by the DHA. The field experiments focus on several strategies to maximise crop yields and nutrient use efficiencies, such as redesigning cropping systems, improving nutrient management to increase crop yield to the biophysical potential and accounting for available nutrients in the field (other than synthetic fertiliser) for crop production. Information from local farmers was collected through questionnaires on crop production. DHA practices may increase crop yields by 13-20% under the IP scenario and by 18-35% under the ISSM scenario. Meanwhile, synthetic fertiliser demand can be reduced by 19-25% under the IP scenario and by 4-14% under the ISSM scenario (Table 1). Synthetic fertiliser demand can be further reduced by taking into account the nutrients available in animal manure. In the present study, we adapted the outcomes of these STBs studies into the IP, ISSM, ISSM-MR and IP-MR scenarios. The explicit assumptions for the DHA scenarios in the present study are described below.

IP scenario

The IP scenario assumes a higher nutrient use efficiency for synthetic fertilisers than GO, and higher yields. These assumptions imply improved nutrient management resulting in more efficient use of fertilisers by crops than currently (Chen et al. 2006). Technologies to improve nutrient use efficiency include using root zone nutrient management to reduce fertiliser use and increasing planting density to increase crop yield. The assumed reduction in synthetic fertiliser use (N/P and Pip) is crop specific (Table 1). Nutrient uptake by crops and domestic animals is 15% higher than in the GO scenario (Chen et al. 2014). In IP scenario, the nutrient demand of crops is assumed to be met by synthetic fertilisers only, as indicated in Eqns S1 and S2.

ISSM scenario

The ISSM scenario assumes higher nutrient use efficiency than the IP scenario. This results from integrated nutrient management, including improved cropping systems in terms of planting dates, combinations of crop types and planting densities (Chen et al. 2011). Synthetic fertiliser use ([N.sub.ISSM] and [P.sub.ISSM]) is assumed to be reduced by 4-14% relative to GO (Table 1). The nutrients used by crops and animal grazing will be 30% higher because of increasing crop yields under ISSM (Chen et al. 2014). Synthetic fertiliser use is calculated using Eqns S3 and S4.

To avoid nutrient losses from animal production, we developed the IP-MR and ISSM-MR scenarios. In these two scenarios, we assumed that crop demand for nutrients would be fulfilled by recycling of animal manure. Synthetic fertiliser would be added only in case animal manure could not fulfil the nutrient demand of crops. In these scenarios, synthetic fertiliser input to the land will be much lower than in the IP and ISSM scenarios.

IP-MR scenario

The IP-MR scenario is as the IP scenario, but assumes that both animal manure and synthetic fertilisers will provide nutrients for crops. The synthetic fertiliser equivalent of animal manure is assumed to be 50% for N and 90% for P (Jensen 2013). Synthetic fertilisers are assumed to be used only when animal manure cannot provide sufficient nutrients for crops ([N.sub.IP-MR]). Synthetic fertiliser use is calculated using Eqn S5 and S6.

ISSM-MR scenario

The ISSM-MR scenario is as the ISSM scenario, but assumes that both synthetic fertiliser and animal manure are nutrient sources for crops. Manure management is as in the IP-MR scenario. Synthetic fertiliser use is calculated using Eqn S7 and S8.

In the GO scenario, nutrient inputs to land are higher and crop yields are lower than in the other four scenarios. These four scenarios can be ranked from small to large reductions compared with GO in term of synthetic fertiliser use as follows: ISSM (N: 10%; P: 7%)< IP (N: 22%; P: 23%)< ISSM-MR (N: 45%; P: 45%)< IP-MR (N: 60%; P: 62%). Similarly, the scenarios can be ranked from low to high crop yields as follows: IP (15%) = IP-MR (15%)<ISSM (30%) = ISSM-MR (30%).

Indicator for coastal eutrophication potential

The Indicator for Coastal Eutrophication Potential (ICEP; Billen and Garnier 2007) indicates the potential for non-siliceous algal growth in coastal waters. The ICEP (in kg C [km.sup.-2] [day.sup.-1]) is calculated from the ratio of N or P to DSi. The ICEP considers the requirements for the growth of diatoms (Redfield ratios of C : N : P : Si = 106:16:1 :20) and is implemented in the Global NEWS 2 model. A high potential for harmful algal blooms is assumed when N and P fluxes are greater than DSi (i.e. ICEP is above zero). A negative ICEP value indicates that silica fluxes in coastal waters are sufficient for diatom (non-harmful) growth (Billen and Gamier 2007).

Results

River export of nutrients

River export of dissolved nutrients increased rapidly from 1970 to 2000 (Fig. 2c-f). DIN exports doubled (from 700 to 1600Gg [year.sup.-1]) and DIN exports quadrupuled (from 20 to 80 Gg [year.sup.-1]; Fig. 2c, d), whereas DON and DOP exports increased by 30% (from 240 to 310 Gg [year.sup.-1]) and 38% (from 130 to 180Gg [year.sup.-1]) respectively (Fig. 2e, f). In the future, dissolved N and P are projected to increase further, especially DIN (by 55% more) and DIP (by 56% more) from 2000 to 2050 under GO. This results from an increase in N and P inputs from animal manure and synthetic fertilisers to land (Figs S2, S3). Synthetic fertilisers and manure are responsible for most of the increases in DIN and DOP (Fig. 2c-f).

Nutrient inputs to the Chinese seas can be reduced by increasing nutrient use efficiencies and crop yields (IP, ISSM, IP-MR and ISSM-MR scenarios). Scenarios assuming efficient recycling of animal manure in crop production (IP-MR and ISSM-MR) can reduce river export of nutrients more than the other two scenarios (IP and ISSM). This is because synthetic fertiliser use is reduced in the MR scenarios (Fig. 2; Fig. S4). As a result, river export of DIN is calculated to be reduced by 15% (ISSM), 10% (IP), 16% (ISSM-MR) and 35% (IP-MR) relative to GO. River export of DOP is calculated to be at least 10% lower (under ISSM) and at most 34% lower (under IP-MR; Fig. 2c). River export of DIP and DON is projected to be reduced, at most, by 24% and 7% (by IP-MR) respectively (Fig. 2d, e).

Reductions in river export of DIN under the ISSM-MR and IP-MR scenarios are higher in southern than northern basins (Fig. 3). For GO, we calculated that DIN fluxes are generally lower than 111 kg[km.sup.-2] year 1 in northern basins (Fig. 3a). ISSM-MR and IP-MR scenarios are calculated to reduce river export of DIN by at least 16% for all basins (except the Hai Ho/ Yongding, Daling He and Yalu). The largest reductions are calculated for the Chang Jiang basin (>36%; Fig. 3b). In southern basins, the reduction in DIN achieved by the IP-MR scenarios (up to 35%) is higher than that seen with the ISSMMR scenarios. For northern basins, such as the Hai Ho/ Yongding, Daling He and Yalu, the reduction in DIN fluxes by the ISSM-MR and IP-MR scenarios is relatively low (<15%). Compared with GO, the dominant source of DIN in southern basins changes from synthetic fertiliser to animal manure under the ISSM-MR and IP-MR scenarios, whereas in northern basins sewage is the dominant source in all scenarios (Fig. 3b).

As for DIN, the largest reductions in DIP were calculated for southern basins (Fig. 4). DIP export by the Huanghe, Chang Jiang and Zhujiang basins is relatively low (<60 kg [km.sup.2] [year.sup.-1]) under GO. For other basins, DIP fluxes are around 90-400 kg [km.sup.2] [year.sup.-1] (Fig. 4a). Under the ISSM-MR and IP-MR scenarios, DIP export is reduced by 5% or less for northern basins and at least 16% for southern basins (Fig. 4b). The largest reduction (>36%) in the ISSM-MR scenario is calculated for the Zhujiang basin (Fig. 4b). The IP-MR scenario is more effective in the Menjiang, Jiulong and Fuchun Jiang basins than the ISSM-MR scenario. In both the IP-MR and ISSM-MR scenarios, the dominant source of DIP in southern basins changes from synthetic fertilisers to sewage. In northern basins, sewage is the dominant source of DIP under these scenarios (Fig. 4c). The southern Mengjiang and Qujiang basins are two exceptions, in which dominant sources will not change (Fig. 4c). The calculated reductions in DON and DOP under the ISSM-MR and IP-MR scenarios are similar for all basins (Figs S5, S6).

From the above, it can be concluded that the effectiveness of DHA to reduce nutrient export by rivers can be ranked from high to low as follows: IP-MR>ISSM-MR>IP>ISSM.

Indicators for coastal eutrophication potentials

The potential for coastal eutrophication in all four DHA scenarios is considerably lower than in GO. The ICEP values are lower than under scenarios with (ISSM-MR and IP-MR) than without (ISSM and IP) advanced animal manure management. Under the ISSM-MR and IP-MR scenarios, six and eight basins have negative ICEP values respectively, indicating a low risk for coastal eutrophication (Fig. 5).

For the South China Sea, risks for coastal eutrophication are high in general under GO. Hanjiang is the only basin with a low eutrophication risk (ICEP < 0) under GO. By implementing the ISSM and IP scenarios, Dongjiang (ICEP=-0.47 kg C Eq [km.sup.2] [day.sup.-1] under the IP scenario) and Hanjiang (ICEP = 4.66 and -5.80 kg C Eq [km.sup.2] [day.sup.-1] under the ISSM and IP scenarios respectively) are calculated to have a low risk of coastal eutrophication (Fig. 5a). Under the ISSM-MR and IP-MR scenarios, the ICEP values of all basins are further reduced, although the ICEP of Zhujiang and Jiulong basins remains positive.

For the Bohai Gulf, DHA scenarios result in reduced ICEP values for all basins compared with GO. Two (HUN/Liao and Luan) of six basins have a low risk for coastal eutrophication under GO (Fig. 5b). Under the ISSM and IP scenarios, although ICEP values for all six basins are calculated to be lower than GO. only the HUN/Liao (-0.08, -0.20 and -0.23 kg C Eq [km.sup.2] day.sup.-1] for GO, ISSM and IP scenarios respectively) and Luan (-2.59 kg C Eq [km.sup.2] [day.sup.-1] for the GO scenario; -2.69 kg C Eq [km.sup.2] [day.sup.-1] for both the ISSM and IP scenarios) basins have a low risk of coastal eutrophication. Under the ISSM-MR and IPMR scenarios, the modelled ICEP of all basins in the Bohai Gulf decreases and the modelled ICEP for Daling He decreases from 0.34 (GO) to -0.04 kg C Eq [km.sup.2] [day.sup.-1] (ISSM-MR and IP-MR scenarios; Fig. 5b).

For rivers draining into the Yellow Sea, the ICEP under the DHA scenarios is lower than for GO. In GO, ICEP values for most basins are generally high and Qujiang (ICEP = -2.56 kg C Eq [km.sup.2] [day.sup.-1]) is the only basin with a low risk of coastal eutrophication. For the ISSM, IP and ISSM-MR scenarios, ICEP values are generally lower than for GO; however, Qujiang is still the only basin with a negative ICEP (Fig. 5c). In the IP-MR scenario, half of the basins have a low risk of coastal eutrophication (Fig. 5c). The ICEP value for the Fuchun Jiang basin decreases from 9.35 (GO) to -0.43 kg C Eq [km.sup.2] [day.sup.-1] under the IP-MR scenario, which is the largest reduction achieved in the DHA scenarios for the Yellow Sea. We also calculated that the ICEP of Mengjiang decreases from a positive value under GO to a negative value under the IP-MR scenario (2.08 and -0.42 kg C Eq [km.sup.2] [day.sup.-1] respectively). The Qujiang basin already has a low risk of coastal eutrophication under GO, and this will be even lower under the IP-MR scenario (-3.1 kg C Eq [km.sup.2] [day.sup.-1]).

In summary, 8 of 16 basins are calculated to have a low risk of coastal eutrophication under the IP-MR scenario (ICEP < 0; Fig. 5). Four of these (Hanjiang, HUN/Liao, Luan and Qujiang) also have low risk for coastal eutrophication potentials under GO, whereas the other four basins (Dongjiang, Daling He, Fuchun Jiang and Menjiang) will change to low risk if the IP-MR scenario is implemented. These findings imply that many basins still have a high risk of coastal eutrophication in the 2050 scenarios, but it is calculated that their ICEP values will be reduced under the DHA scenarios.

Discussion

Earlier studies analysed nutrient use efficiencies for China (Ma et al. 2013) and the effects of DHA on crop yields by long-term field experiments and questionnaires in over hundreds of counties in China (Zhang et al. 2011, 2016; Chen et al. 2014). In addition, Qu and Kroeze (2010) and Strokal et al. (2014) quantified river export of nutrients for China, and the sources of these nutrient. However, no studies linked the effects of DHA on coastal nutrient enrichment. The present study is the first modelling study, using the Global NEWS 2 model, of the effects of DHA on coastal eutrophication. The results indicate that DHA may lead to lower nutrient inputs to rivers and coastal waters because of lower synthetic fertiliser demand, and that this will likely reduce the potential for coastal eutrophication. Despite the assumptions and simplifications, we believe that the present study is innovative and relevant; it adds new information to existing knowledge and increases our understanding of the potential of DHA to reduce nutrient inputs to rivers from agriculture, and thus to reduce coastal eutrophication.

Potential of DHA management to reduce coastal eutrophication

The results of the present study are in line with other studies reporting large increases in fertiliser application in past decades (Ma et al. 2008, 2013; Sims et al. 2013). In our study area, the total N and P inputs from synthetic fertilisers and animal manure to land increased by a factor of 4 for 16 river basins (from 8 to 32 Tg N, and from 1 to 4 Tg P) from 1970 to 2000. In GO, total N and P inputs may increase further in the future (Fig. 2a, b). Ma et al. (2012) modelled comparable increases in N fertiliser use (from 12 to 34 Tg) and P fertiliser use (from 1 to 5 Tg) from 1980 to 2005 in China.

In the present study, river export of total dissolved nitrogen in the past (Fig. 2c, e) compared reasonably with measurements in the 1980s (Duan et al. 2000). Gao et al. (2015), Wang et al. (2014) and Li et al. (2014) also provide evidence of increasing DIN in Chang Jiang and concluded, as we have, that this increase in river export of nutrients may have been caused by an increase in fertiliser inputs to land. In addition, we calculated that synthetic fertiliser and animal manure are responsible for 80% of the increased nutrient export to seas in the past (Fig. 2c J).

The results of the present study imply that DHA may lower river export of DIN and DON because it increases the nutrient use efficiency in agriculture. In the GO scenario for 2050, modelled river export of DIN and DON increased to 2.5 and 0.4 Tg [year.sup.-1] respectively, and synthetic fertiliser and animal manure are major sources of river export of DIN and DON in the GO scenario for 2050. Under the IP-MR scenario, synthetic fertiliser inputs to land are calculated to be 56% lower (10 Tg [year.sup.-2]) than GO (23 Tg [year.sup.-2]). In addition, these inputs result in a lower river export of DIN and DON under DHA scenarios (i.e. IP, ISSM, IP-MR, ISSM-MR scenarios). Thus, IP-MR lowers the coastal eutrophication potential for rivers with synthetic fertiliser and animal manure as major sources of river export of nutrients. We calculated that in GO 12 basins have a high risk of coastal eutrophication (Fig. 5). DHA scenarios may reduce the risk of coastal eutrophication to low for the Dongjiang, Daling He, Fuchun Jiang and Menjiang rivers (Fig. 5).

The present study reveals that among the DHA scenarios, those with advanced animal manure management (i.e. ISSM-MR and IP-MR) are more effective than the other two scenarios in reducing coastal eutrophication potentials. Differences between ISSM and IP or between ISSM-MR and IP-MR in reducing river export of nutrients are relatively small. We argue that ISSM-MR is the most promising for the future in China because, in assuming that ISSM crop yields are higher than in IP, the ISSM-MR predicts a future in which food demand is met and coastal eutrophication potentials are simultaneously reduced.

We used the 2010 version of the Global NEWS 2 model for our analyses, and the GO scenario as a reference. This implies that we do not account for direct inputs of manure to rivers, which has been current practice in China in animal production in recent years (Strokal et al. 2016). We implicitly assume that by 2050 this practice will have stopped, and that all manure is returned to the land. So, manure is treated as a diffuse source of nutrients in rivers only. We realise that we may be underestimating nutrient inputs from animal production to rivers compared with the current situation.

Increasing future nutrient use efficiency of animal manure

In China, large amounts of synthetic fertilisers are applied to crop land. The N applied to cereal crops in 2004 was reported to be 22 Tg, with 70% of this input from synthetic fertilisers (Ma et al. 2008). As an alternative, N losses from animal manure to the environment can be reduced by using manure as an organic fertiliser for crop production (Ma et al 2010). The mineral (or synthetic) fertiliser equivalency (MFE) value describes how much of mineral fertilisers can be replaced by animal manure (Schroder 2005; Jensen 2013). Future options for increasing the nutrient use efficiency of animal manure should take into account increasing animal manure recycling in cropland and increasing the M FE of animal manure (Jensen 2013).

To better manage animal manure, techniques such as low-protein animal feed, barn adaptation, covered manure storage and air purification can be applied to avoid N losses to the environment (Pellini and Morris 2002). Nutrient losses can be further reduced by implementing other nutrient management options, such as preplant nitrate testing as a guide to reduce the N application rate.

It is possible to increase the MFE by improving the method of fertilisation with animal manure. N losses through ammonia emissions are generally low for slurry when it is applied using splash plates, trailing hoses (band spreading), trailing shoes and injection application (Jensen 2013). Thus, the nitrogen MFE of animal manure can be increased. Some preprocessing of animal manure, such as acidification of the slurry-type of animal manure (adding sulfuric acid to animal manure; Sorcnsen and Eriksen 2009) and using modern solid manure spreaders, may also help increase the MFE of animal manure (Thomsen 2004). By integrating more technologies into nutrient management in the Chinese agricultural sector, the effects of agriculture on coastal eutrophication are likely to be further mitigated.

In China, animal production is often separated from crop production (Ma et al. 2012). Animal production has become highly industrialised in past years and crop production is managed on the small farm scale. The technologies mentioned above can be applied at the farm level. To evaluate the performance of these technologies in improving coastal water quality, it is important to link the Global NEWS 2 model with farm-level practices, especially with regard to the management of animal manure. Therefore, it is necessary to consider nutrient flow at the farm level to further evaluate the performance of nutrient management strategies in reducing nutrient enrichment in coastal seas.

In addition to the technical feasibility of recycling animal manure, implementing improved animal manure management is a challenge in practice, requiring support from agronomists and the Chinese government. The government needs to support a reduction in synthetic fertiliser use and an increase in the use of organic fertilisers. It is important to constrain the use of synthetic fertilisers and to provide incentives to facilitate the recycling of manure as an organic fertiliser, as well as to establish transportation systems for organic fertiliser between regions. Scientists can help establish advisory boards to educate farmers about fertilisation. In 2015, the Chinese government launched a new policy on restricting synthetic fertiliser use (Ministry of Agriculture 2015). As indicated above, the number of STBs has increased quickly in China (Zhang et al. 2016). Therefore, we see a high potential to implement DHA with recycling of animal manure in practice in the future.

Uncertainties

One of the largest uncertainties in the present study is our description of the future. We used the reference scenario for 2050 from the IAASTD to estimate the percentage of synthetic fertilisers applied to different land (cropland, legumes, grasslands, wetland rice) in provinces of China (Bouwman et al. 2013). We aggregated data from the provincial scale of the IAASTD to the basin scale of the Global NEWS 2 model. This was necessary to calculate the reduction in fertiliser use in each basin under DHA management. We realise this aggregation may affect basin-scale results. In addition, the DHA program is currently only applied to cereal crops, but we assume the program will be expanded to all crops in the future. However, we believe that the final results are not greatly affected by these uncertainties.

The Global NEWS 2 model is a steady state model, which means the accumulation of nutrients in soils and sediments was not explicitly accounted for. This creates an error for diffuse sources of dissolved forms of P in particular, because P tends to accumulate in soils and sediments. For point sources of P in rivers and N the error is likely smaller. Most N that is not taken up by crops likely denitrifies to gaseous forms. Nevertheless, we realise that these omissions may affect the results for specific years.

Other uncertainties are associated with source attributions in the Global NEWS 2 model. The model considers animal manure as a diffuse source of nutrients in rivers (Mayorga et al. 2010), ignoring direct discharges of animal manure to surface waters. However, in reality, the direct input of animal manure to rivers is relatively large (Ma et al. 2012). Thus, we may be underestimating water pollution. DHA has been tested for major cereal crops (Chen et al. 2006; Zhang et al. 2011; Xia et al. 2013), but studies of DHA on other crops are limited. Nevertheless, we assume DHA will be applied to all crops by 2050. Our simplification of applying DHA to all crops may lead to over- or underestimation of the results of nutrient export by rivers.

Conclusion

In the present study we analysed the effects of improved nutrient management on N and P export by rivers in China, and the associated coastal eutrophication for the period 1970-2050. Between 1970 and 2000, the total N and P inputs from animal manure and synthetic fertiliser to rivers tripled. We used the Global NEWS 2 model to simulate river export of nutrients to coastal seas. The annual river export of dissolved N and P increased two- to fivefold from 1970 to 2000; these river exports may increase further in the future. In our reference scenario (GO), modelled river export of DIN, DIP and DOP increased by over 50% between 2000 and 2050. Synthetic fertiliser and animal manure are dominant sources of DIN, DIP and DOP in rivers.

We modelled the effects of increasing nutrient use efficiencies and crop yields in agriculture (ISSM and IP scenarios) and advanced animal manure management (ISSM-MR and IP-MR scenarios). These scenarios indicate that river exports of DIN and DIP can be reduced by over 30% (IP-MR) relative to GO. Thus, the risks of coastal eutrophication are much lower compared with GO. In scenarios without advanced manure management (ISSM and IP) most basins still have high risks of coastal eutrophication. In scenarios assuming advanced manure management (ISSM-MR and IP-MR), six and eight basins have low ICEP values, indicating a low risk of coastal eutrophication. We conclude that reducing synthetic fertilisers alone is not enough to reduce the risks of coastal eutrophication. DHA can effectively reduce nutrient loadings in rivers and coastal seas only if it is accompanied by recycling of manure in crop production (as in the ISSM-MR and IP-MR scenarios). Achieving this would require changes in current practices in animal and crop production in China. Future studies could focus on how to realise this in practice.

http://dx.doi.org/10.1071/SR17035

Supplementary material

Additional information on the Global NEWS 2 model and associated equations, detailed steps of aggregate data from provincial level to basin level, details of assumption and equations for each scenario, and additional results of the quantity and special distribution of fertiliser and animal manure input to land, and river export of DON and DOP under different scenarios are available from the Journal's website.

Acknowledgements

The authors acknowledge Chinese National Basic Research Program (2015CB150405), The National Key Research and Development Program of China (2016YFD0800106), Program of International S&T Cooperation (2016YFE01103100), The China Exchange Program (CEP) of the Royal Netherlands Academy of Arts and Sciences (KNAW--Koninklijke Nederlandse Akademie van Wetenschappen, 530-5CDP27), President's International Fellowship Initiative, President's International Fellowship Initiative (PIFI) of the Chinese Academy of Science (2015VEA025), Distinguished Young Scientists Project of Natural Science Foundation of Hebei (D2017503023) and the Hundred Talent Program of the Chinese Academy of Science for providing financial support for this research to be conducted.

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Ang A. Li (A,B), Maryna M. Strokal (B), Zhaohai Z. H. Bai (A), Carolien C. Kroeze (B), Lin L. Ma (A,D), and Fusuo F. S. Zhang (C)

(A) Key Laboratory of Agricultural Water Resources, Center for Agricultural Resources Research, Institute of Genetic and Developmental Biology, The Chinese Academy of Sciences, 286 Huaizhong Road, Shijiazhuang 050021, Hebei, China.

(B) Water Systems and Global Change Group, Wageningen University & Research, PO Box 47, 6700 AA, Wageningen, The Netherlands.

(C)College of Resources and Environmental Sciences, China Agriculture University, Beijing 100193, China.

(D) Corresponding author. Email: malin1979@sjziam.ac.cn

Received 28 March 2017, accepted 14 June 2017, published online 18 August 2017

Caption: Fig. 1. The 16 river basins that drain into Chinese coastal waters (only basins with five or more grid cells of 0.5[degrees] longitude x 0.5[degrees] latitude are selected). Basin boundaries are from the Global Nutrient Export from Watersheds 2 (NEWS 2) model (Mayorga et al. 2010) and the national boundary is from the Global Administrative Areas (2012). (Fig. 1. source: ESR1, GEBCO, NOAA, National Geographic, DeLorme, HERE, Geonames.org, and other contributors.)

Caption: Fig. 2. (a) Nitrogen and (b) phosphorus inputs to land from synthetic fertilisers and animal manure in 1970, 2000 and for the five scenarios modelled for 2050. GO, Global Orchestration scenario; IP, improved practice; ISSM, integrated soil--crop system management; IP-MR, IP with advanced manure management (i.e. manure recycling); ISSM-MR, ISSM with advanced manure management (see text for details). (c-f) Modelled river export (total loads) of dissolved inorganic nitrogen (DIN; c), dissolved inorganic phosphorus (DIP; d), dissolved organic nitrogen (DON; e) and dissolved organic phosphorus (DOP; f) for 1970, 2000 and 2050. Source categories include nutrient fixation and deposition (Fix/Dep), leaching and deposition. Data are from the Global Nutrient Export from Watersheds 2 (NEWS 2) model (Mayorga et al. 2010) and the International Assessment of Agricultural Knowledge, Science, and Technology for Development (1AASTD) baseline scenario (Bouwman et al 2009).

Caption: Fig. 3. (a) Modelled river export of dissolved inorganic nitrogen (DIN) in 2050, (b) percentage DIN reduction by the 'Double High Agricultural" scenarios in 2050 compared with the Global Orchestration (GO) scenario and (c) dominant sources of DIN for the GO, improved practice with advanced manure management (IP-MR) and integrated soil--crop system management with advanced manure management (ISSM-MR) scenarios in 2050. For detailed scenario descriptions, refer to the text. Data are from the Global Nutrient Export from Watersheds 2 (NEWS 2) model (Mayorga et al. 2010), the International Assessment of Agricultural Knowledge. Science, and Technology for Development (IAASTD) baseline scenario (Bouwman et at. 2009) and the present study.

Caption: Fig. 4. (a) Modelled river export of dissolved inorganic phosphorus (DIP) in 2050, (b) percentage DIP reduction by the 'Double High Agricultural' scenarios in 2050 compared with the Global Orchestration (GO) scenario and (c) dominant sources of DIP for the GO, improved practice with advanced manure management (IP-MR) and integrated soil--crop system management with advanced manure management (ISSM-MR) scenarios in 2050. For detailed scenario descriptions, refer to the text. Data are from the Global Nutrient Export from Watersheds 2 (NEWS 2) model (Mayorga et al. 2010), the International Assessment of Agricultural Knowledge, Science, and Technology for Development (IAASTD) baseline scenario (Bouwman et al. 2009) and the present study.

Caption: Fig. 5. Indicator for coastal eutrophication potential (ICEP) values for 2050 for river basins draining into the (a) South China Sea, (b) Bohai Gulf and (c) Yellow Sea with the five scenarios modelled in the present study. GO, Global Orchestration scenario; IP, improved practice; ISSM, integrated soil-crop system management; IP-MR, IP with advanced manure management (i.e. manure recycling); ISSM-MR, ISSM with advanced manure management (see text for details). Note, ICEP < 0 indicates a low risk of coastal eutrophication.
Table 1. Technical potential of 'Double High Agricultural'
management to reduce fertiliser input to Chinese crop production and
increasing yields of rice, wheat and maize

Unless indicated otherwise, data are shown as the mean [+ or -] s.d.
Data are from Chen et al. (2014) and the present study. CP, current
practice (assumed to be Global Orchestration (GO)); IP, improved
practice; ISSM, integrated soil-crop management; n.a., not
applicable; Global NEWS 2, Global Nutrient Export from Watersheds 2

Crop    Land use in     Treatment        Yield          % Yield
        Global NEWS 2               (Mg [ha.sup.-1])    increase

Rice    Wetland rice    CP (GO)     7.2 [+ or -] 1.1      n.a.
                        IP          8.1 [+ or -] 1.1        13
                        ISSM         8.5 [+ or -]1.2        18

Wheat   Upland crops    CP (GO)     7.2 [+ or -] 1.4      n.a.
                        IP          8.3 [+ or -] 1.7        15
                        ISSM        8.9 [+ or -] 1.7        24

Maize   Upland crops    CP (GO)     10.5 [+ or -] 1.6     n.a.
                        IP          12.6 [+ or -] 2.2       20
                        ISSM        14.2 [+ or -] 2.6       35

                                    Synthetic fertiliser
                                    (kg N [ha.sup-1])

Crop    Land use in     Treatment
        Global NEWS 2                   N            P

Rice    Wetland rice    CP (GO)        181          n.a.
                        IP             146          n.a.
                        ISSM           162          n.a.

Wheat   Upland crops    CP (GO)        257          n.a.
                        IP             192          n.a.
                        ISSM           220          n.a.

Maize   Upland crops    CP (GO)        266          n.a.
                        IP             214          n.a.
                        ISSM           256          n.a.

                                    Technical potential to
                                    reduce fertiliser
                                    (% reduction of CP (A))

Crop    Land use in     Treatment
        Global NEWS 2                   N            P

Rice    Wetland rice    CP (GO)        n.a.         n.a.
                        IP               19           19
                        ISSM             10           10

Wheat   Upland crops    CP (GO)        n.a.         n.a.
                        IP               25           25
                        ISSM             14           14

Maize   Upland crops    CP (GO)        n.a.         n.a.
                        IP               20           20
                        ISSM              4            4

(A) Percentage decreases in P are assumed to be the same
as those in N.
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Author:Li, Ang A.; Strokal, Maryna M.; Bai, Zhaohai Z.H.; Kroeze, Carolien C.; Ma, Lin L.; Zhang, Fusuo F.S
Publication:Soil Research
Article Type:Report
Geographic Code:9CHIN
Date:Aug 1, 2017
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