**1.3. Sustainability and productivity**

The second approach for assessing soil and water impacts of bioenergy systems, and the sustainability of biomass production, is dependent on soil quality monitoring. This approach was developed as a means of evaluating the effects of forestry and agricultural management practices on soil functions that might affect site productivity [13, 14]. A number of soil physical, biological and chemical parameters, which have linkages to soil productivity have been proposed as forming a minimum monitoring set. The way forward relative to assessing soils impacts and the sustainability of biomass production systems rests with proactive proper soil management and not reactive monitoring for screening the condition, quality, and health of soils relative to sustaining productivity [15–17]. Evaluation of soil condition thus would lead to a time-trend analysis that can in turn be used to assess the sustainability of land management practices and bioenergy programs. Even though sustainability is the stewardship goal of land management, more specific definitions of its goals and attributes is often complex and open to considerable interpretation [18, 19]. Many scientists have attempted to answer the "what," "what level," "for whom," "biological or economic," and "how long" questions of sustainability. However, there is no absolute definition of sustainability, and that it must be viewed within the context of the human conceptual framework, societal decisions on the state of ecosystem to be sustained, and the temporal and spatial scales over which sustainability is to be judged [18]. In short, this approach is loaded with considerable uncertainty and lack of consensus.

**2.1. Annual agricultural crops**

and temporal scales for assessment [21].

**2.2. Perennial and semi-perennial crops**

moisture deeper in soil profiles [27].

or more intensively managed tree crops.

**2.3. Forest woody biomass**

The cultivation of conventional annual crops as bioenergy feedstocks affects soil and water resources similar to crop cultivation for food and livestock feed. Water withdrawals and the effects of agrochemicals must be carefully managed to avoid human health impacts, water quality degradation, and damage to ecosystems [20]. As in other agricultural and forestry activities, the adoption of BMPs is crucial to minimizing the risk of water quality impacts and promoting sustainable resource use. Assessing BMPs and their effectiveness further requires defining appropriate water quality expectations, determining what site conditions limit BMP effectiveness, and identifying the specific watershed characteristics and appropriate spatial

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Extensive root systems, long-term soil cover and protection, and reduced need for tillage and weed suppression, give semi-perennial crops excellent choices for bioenergy feedstocks. Crops such as sugarcane, perennial grasses like switchgrass, *Miscanthus* spp. and elephant grass, and trees grown in short rotations tend to have lower water quality impacts than conventional crops [22–24]. While many perennial crops considered for bioenergy have relatively high water use efficiency, their total water requirements can also be relatively large. Such crops are ideally suited to areas with high water availability and flows where water quality can be easily managed [25]. For example, one analysis indicated that that *Miscanthus* spp. could replace 50% of corn acreage in most areas of the Midwest US without adversely affecting the hydrologic cycle. In drier regions, *Miscanthus* spp. should be limited to 25% of the area [26]. Additionally, it has been suggested that the use of perennial grasses may increase seasonal evapotranspiration (ET) compared to grains due to the access of these grasses to

Forests provide important regulation of both water quality and seasonally available water quantity in most large watersheds. Forest bioenergy systems are judged compatible with maintaining high-quality water supplies in forested catchments. This general statement is true as long as BMPs that are designed for environment and resource protection, and include nutrient management principles, are followed [28–30]. While short term water impacts, including increased sediment, nitrates, phosphates, and cations can occur, there is no evidence of long term adverse impacts in forest catchments subject to normal management operations [12]. However, more research is needed to guide BMPs concerning special activities in forest management (e.g. stump extraction, weed control, and forest fertilization [29, 31]. Quantitative water flows in forest stands are affected if stands are subject to operations involving significant basal area reductions. But since a forest estate typically is a mosaic of stands of different ages, where only a small share of all stands are harvested in a particular year, water flow regimes on the larger landscape level typically are not affected significantly by stand level operations. Exceptions occur where forests are replaced with other land covers
