Agriculture Reference
In-Depth Information
Biomass Nutrient Stores for Vines Compared to Other Vegetation Types
Table 4.3
Nutrient Store (kg/ha)
Vegetation type
N
K
Ca
Mg
Nonirrigated 18-yr-old vines (Cabernet
51
57
Sauvignon), 1120 vines/ha, at harvest
(including roots)
Irrigated 10-yr-old vines (Chenin Blanc),
1680 vines/ha
at bud burst (including roots)
215
124
at harvest (including roots)
366
423
Dry grassland (including roots and litter)
46
105
13
24
Mature rain forest (including roots and litter)
2040
910
2670
350
Source:
After Williams and Biscay (1991), Mullins et al. (1992), and White (1997)
factors such as the variety, planting density, and vigor of the vines, as well as cul-
tural practices (irrigation, pruning, leaf pulling, and fruit thinning). For example,
the measured biomass C in a nonirrigated vineyard (1120 vines/ha) at fruit ma-
turity was 5.2 t C/ha (all plant parts) (Williams and Biscay 1991). Of this bio-
mass C, only about 15-20% would recycle annually. Examples of biomass stores
for some of the macronutrients in vines and other vegetation types are given in
table 4.3.
Cover crops in vineyards add considerably to the biomass store and are an
important source of N if a legume is present. All biomass (above- and below-
ground) is important in nutrient cycling, irrespective of the size of the store, be-
cause the residues of living organisms and their excreta are the substrate on which
the soil reducers and decomposers feed (section 2.3.2). It is difficult to estimate
the size of the root biomass, because roots are intimately associated with the soil
and can extend to a depth of several meters. Generally the root store amounts to
50-100% of the aboveground store.
Of the soil organisms, earthworms and the microbial biomass are the two
most important nutrient stores. The microbial biomass has been identified as the
“eye of the needle” through which all the C returned to the soil must eventually
pass. Within this microbial biomass, the mean C:N ratio of bacteria is ca. 4 and
that of fungi is ca. 10 (White 1997), so the biomass is also an important store of
N (100-500 kg N/ha). Microbial biomass P values range from ca. 25 to 100 kg
P/ha. A few species of microorganisms are exceptional in being able to reduce mo-
lecular N
2
to NH
3
and incorporate it into amino acids for protein synthesis
through a process called
biological N
2
fixation
.
Biological N
2
Fixation
In agro-ecosystems, wherever legumes such as clover or lucerne are grown, the in-
put of biologically fixed N is an important component of the N cycle. The N
2
fixed by legume cover crops can provide a significant input of N in vineyards.
The microorganisms responsible for fixation live either independently in the soil
as
free-living
organisms or in
symbiotic association
with plants.
Symbiotic N
2
fixation
. Symbiosis denotes the cohabitation of two unrelated
organisms that benefit mutually from the close association. The most important
4.2.2.1
