Assessment of Grassland Stands in Respect of Nutrient Yields and Safety Net Functions for 2002 / 2003 Safety Net Functions for 2002 / 2003

Higher WUEbm in stand I and II in 2003 is assessed as a positive criterion for grassland stands, because global climatic change likely forces water saving management practices.

Stands with high NUEbm may provide high productivity at low fertilizer inputs. In this concern, stand I (H. lanatus + A. elatius) showed slight advantages compared to H. lanatus monoculture and H. lanatus + P. lanceolata co dominated grassland stands in 2002. The N yields were tenden-tiously higher and loss/yields-ratios were lower in stands dominated by P. lanceolata.

In 2003, only stand II (H. lanatus + G. pratense) differed from the other stands with a lower NUEbm. Low NUEbm and high loss/yield-ratios classify stand II as a fertilizer consuming grassland stand with higher risk of N leaching compared to the other stands.

Our grasslands only received low compensation fertilization (10 to 11 g N m^-2 yr^-1). Taking rec-ommended fertilizer application for European meadow grasslands (30 to 70 g N m^-2 yr^-1; W HITE-HEAD, 1995) into consideration, stand II might have shown intolerable nitrogen concentrations (›

50 mg NO3-N l^-1) and loss in seepage after such fertilizer applications.

Stand I (A. elatius + H. lanatus) had higher NUEbm, high N yields and low loss/yield-ratios. Hence in concern of demand, yield and safety net functions for N grass dominated stand I was the grassland stand with the best performance in 2003.

P. lanceolata monocultures (stand III) showed higher N yields than grass/herb mixtures (stand IV/V). The effect of P. lanceolata contribution on N use parameters had decreased since N con-tents in 2003 were similar to grass species concon-tents (Appendix, Table VI/VII).

In 2002, stands with a higher herb contribution (III-V) showed lower KUEbm, MgUEbm and CaUEbm. These findings were associated with higher yields in K, Mg and Ca. Thus in concern of safety net functions for K, grassland stands higher herb contribution were classified as more suit-able. Since the main herb biomass was provided by P. lanceolata, the base cation use of stand III-V was due to species traits of P. lanceolata to a major extent.

In contrast to 2002, the grass dominated stand I had higher KUEbm than the other grassland stands in 2003. Due to higher biomass production, stand I showed highest K yields and tenden-tiously lower loss/yield ratios than the other stands. In concern of K demand, grass dominated stands showed advantages.

In 2003, the highest MgUEbm indicated lower Mg demands for grass stand I. The Mg yields were low and the loss/yield-ratio was high. Therefore, high yields and low loss/yield-ratios classified stand III (P. lanceolata) as most suitable in concern of safety net functions for Mg in 2003.

Grass dominated stands I and II had higher CaUEbm in 2002. These stands showed considerably lower Ca yields and higher loss/yield-ratios. Stand III (H. lanatus + P. lanceolata) showed a lower CaUEbm, higher Ca yields and lower loss/yield-ratios. Thus stand III was classified as most suit-able in concern safety net functions for Ca in 2002.

Lower K, Mg and Ca use efficiencies were found in H. lanatus + P. lanceolata stands (III-V) in 2002 and in P. lanceolata + A. elatius in accessory of subordinates (IV-V). Since these features appeared to be most pronounced in monocultures, they are most likely due to species traits of P.

lanceolata. At considerable contribution to stand biomass (› 50 %), the presence of P. lanceolata determined the base cation use of our grassland stands. Since G. pratense and T. officinale shared high base cation contents, they also contributed to base cation yield and safety net func-tions. However, their contribution was limited due to low biomass productivity.

In 2003, higher biomass production led to considerably higher K yields and lower loss/yield ra-tios despite of higher KUEbm in stand I (A. elatius + H. lanatus). The grass species combination likely facilitated higher K acquisition compared to stand II by its homorhizal root system.

BERGMANN (1992) confirmed higher K acquisition for grass species compared to herb species due to their homorhizal root systems. The presence of allorhizal belowground system of G. prat-ense in stand II obviously impaired K acquisition and led to higher loss/yield ratios than in stand I.

DAEPP ET AL. (2001) reported N aboveground yields to 10 g N m^-2 yr^-1 in a Lolium perenne two-cut regime. SCHILS ET AL. (1999) gave aboveground yields ranging from 9.1 to 13.6 g N m^-2 yr^-1 in a Lolium perenne two-cut regime with application of 8 g N m^-2 yr^-1. DIERSCHKE >BRIEMLE

(2002) gave aboveground yields of extensively used grasslands of 10 g N m^-2 yr^-1 for a two-cut regime at compensation fertilization. LFL (2003) confirmed aboveground yields ranging from 6.5 to 7.8 g N m^-2 yr^-1. N aboveground yields of similar grassland stands of the former BIODEPTH-site ranged from 2 to 15 g N m^-2 yr^-1 (NEßHÖVER,2005).

Nutrient accumulation of our grassland stands (Table 22, 23), included nutrients in above and belowground biomass. In 2002 aboveground accumulation accounted for 70-75 % of Nstand of the grassland stands, whereas the contribution of aboveground accumulation to Nstand decreased for all stands in 2003, ranging from 48 to 56 % (Appendix, Table III). Regarding this, the mean N accumu-lation in our grassland stands was rated medium to high for 2002 and 2003.

In 2002, stand III and IV (H. lanatus + P. lanceolata) showed somewhat higher N yields than stand I and II (H. lanatus). This finding reflects higher N accumulation to aboveground biomass due to P.

lanceolata contribution. In 2003, higher N yields in biomass of stand I and II were very likely due to enhanced mineralisation (also 3.1.3.2) of H. lanatus detritus (Figure 22). However, in stand III-V detritus input was almost equal. Stand III (P. lanceolata) showed tendentiously higher N yields than stand IV - V. Hence, higher functional diversity in these stands did not lead to enhanced N accumulation. This finding hints at increased competition for nitrogen in comparison to stand III. A decline in the contribution of aboveground N accumulation to Nstand likely indicated increasing belowground competition between plants in the grassland stands. Since this decline was found for all grassland stands in 2003, competitive rather than complementary relations between spe-cies were indicated, irrespective of functional group controlled N yields in our grassland stands.

In both years, grass dominated stands showed higher use efficiencies for K, Mg and Ca. Hence grass species showed lower demands in these base cations for biomass production. In concern of safety net functions, the performance of grass dominated stands was quite poor, due to higher demands grass/herb-mixtures showed lower loss/yield-ratios and thus higher safety net functions.

In 2003, stand I showed despite of higher KUEbm considerable K yields and low loss/yield-ratios.

This finding was mainly attributed to a higher biomass production compared to the other grass-land stands.

H. lanatus + A. odoratum+ P. lanceolata + R. acris

Implications of different Fe nutritional strategies on the release of rhizodeposits were investi-gated using four grassland species out of the pool of species used for the lysimeter experiments.

The 1^st biomass harvest of swards established in pots took place three and the 2^nd harvest five months after establishment. Fe was re supplied three weeks after the 1^st harvest.