Nitrogen Transfers and Transformations in Row-Crop Ecosystems - The Ecology of Agricultural Landscapes - page 220

press), or 92 kg N ha −1 yr −1 (Table 9.4). This percentage agrees closely with recent
reviews (Herridge et al. 2008, Salvagiotti et al. 2008).
Cover crops are a second major source of BNF in the MCSE Reduced Input
and Biologically Based systems (Table 9.4). Wilke (2010), based on 15 N natural
abundance, estimated that BNF in red clover accounted for 55 and 72% of its total
aboveground N in these respective systems. BNF in the successional communities
of the MCSE has not been measured but based on legume biomass, and N content
is likely ~10 kg N ha −1 yr −1 in the Early Successional community and negligible in
the Mid-successional system. No more than 7 kg N ha −1 yr −1 is likely fixed in the
mature Deciduous Forest system (Cleveland et al. 1999).

Nitrogen Deposition
Nitrogen oxides (NO and NO 2 , referred to as NO x ) are emitted from industrial
combustion and other sources, including agricultural soils. Oxidation of NO x
results in increased atmospheric N deposition to ecosystems, which in turn causes
higher regional watershed N fluxes (e.g., Jaworski et al. 1997), increased soil N 2 O

Table 9.4. Biological N fixation (BNF) by legumes, aboveground N, and percentage
of aboveground N derived from BNF in annually harvested MCSE systems. a

Variable Annual Crops Perennial Crops d

Conventional No-Till Reduced

Input

Biologically
Based

Alfalfa

Legume aboveground N (kg N ha −1 yr −1 ) b
Soybean 159 184 170 166

Red clover 56 63

Alfalfa 215

Total aboveground N (kg N ha −1 ) c 342 383 398 341 215

BNF (kg N ha −1 yr −1 ) b
Soybean e 92 107 99 96

Red clover f 31 45

Alfalfa g 157

Aboveground N met by BNF (%) 27 28 33 41 73

a Mean, n = 6 replicated plots (SE not shown).
b N in legume aboveground biomass and N from BNF per year (de facto per 3-year rotation for soybean and red
clover) for each N-fixing crop (as described below) averaged over six rotations (1993-2010).
c N in grain and stover/straw of corn, soybean, and wheat plus N in harvests of the cover crops red clover (1995-2008)
and cereal rye (2006-2009) per 3-year rotation averaged over six rotations (1993-2010).
d For alfalfa, legume aboveground N, total aboveground N, and N from BNF determined per year averaged over
1993-2010 (15 years with crop present).
e BNF of soybean estimated as 58% of the total aboveground N uptake (Gelfand and Robertson, in press; Salvagiotti
et al. 2008).
f fBNF of red clover estimated from the percentage of BNF-derived N (55 and 72% for Reduced Input and Biologically
Based systems, respectively; average of fall 2007 and spring 2008 harvest data from Wilke 2010; Figure 9).
g BNF of alfalfa estimated as 73% of the total aboveground N uptake (Yang et al. 2010).